The way I did it last year (I had enough mining income to warrant and enough ordinary income that I am in one of the classes the IRS likes to audit) is as follows.  The CPAs in the room can tell me how off base I was. 

Mining output = Ordinary Income reported to IRS on Line 7 of 1040 by calculating the value on the date (I use Mt Gox weighted average unless they are unavailable in which case I use BTC-E value half way between daily high and low) it was transferred into my wallet.

The value calculated above provides my cost basis of the BTC I own (I never buy on exchanges, but if I did this would be there as well).  When I sell them the profit from my cost basis (if any) are calculated as the difference between my cost basis and the sales price.  Thus I must keep records of the individual cost basis of each BTC I receive separately (and thus it pays to make transactions large and few) and allocate them individually to each sale.  This value is reported as Cap Gains on Schedule D and 1040 Line 13 to the IRS and I pay cap gains (currently short term as I have not held any BTC long enough to qualify as long term) tax on that value.

Anyone not waiting for multiple confirmations and not listening on the network for double spend attempts on transactions this large deserves what they get.

Not sure why everyone is so panicked.  We only orphaned 25 blocks and the only danger was that you would accept the coins minted in those blocks (all transactions using other coins would eventually end up in the other chain as well).  If we just followed the network rules and waited 100 blocks to accept minted coins then there was actually no danger at all.  What am I missing?  Seems to me like the devs and the pools worked together to quickly address the issue and that there is a plan to move forward with a permanent fix. Just my $.02.

I've come up with a system for a blockchain based stock exchange and would like to run it by you guys before I get too far down the road on it.  The idea is this:

1.  The private owner of an enterprise decides to take it public.
2.  The owner creates a vanity address for the IPO with the first few characters after the 1 representing the ticker symbol of the stock and transfers into it a number of bitcoins representing the total number of outstanding shares that will be issued (in Satoshis) and announces the IPO publicly.
3.  Any person wishing to purchase the stock also creates a vanity address with the same first characters as the owner and then negotiates a sale with the owner at a reasonable price.  The transaction is then recorded as follows
     A.  The buyer transfers some number of Bitcoins from any arbitrary address to the owner of the stock
     B.  The owner then transfers the agreed number of shares (denominated in Satoshis) from the vanity address of the IPO to the vanity address of the buyer.

Transactions between owners of the stock later take place in a similar manner and only transactions between vanity addresses with the same first characters actually are considered to transfer ownership of the stock.  The fact that all such transactions are in the blockchain makes them audit-able for anyone.  Obviously there will be opportunities for third party services to escrow the transactions and to advertise the existence and market for various stocks (this is where I intend to come in), but the information about the ownership of the underlying stocks is tied into the blockchain and is therefore not owned by any one individual.  Let me know what you guys think.

I was thinking it would be possible if both transactions were in the same block. 

I'm wondering how the network would handle mutually dependent transactions (I'm considering a blockchain based stock exchange).  The question is how the network would handle transactions where the output of each is used as the input (or part of the input of the other.  My idea for the stock system will be in the econ area, but I would like some feedback on the following scenario and what to expect from the network.

Address 1 has 20BTC to start
Address 2 has .1BTC to start
at roughly the same time
Address 1 sends 20.1BTC to Address 2 and
Address 2 sends .2BTC to Address 1

Address 1 has .1BTC to end
Address 2 has 20BTC to end

but these two transactions are intertwined and one is only valid if the other is as well.

I'm wondering how the network would treat such transactions

The only realistic problem I can see with fewer miners is that they may collude to increase TX fees.  If a large proportion (> 50%) decided they would no longer include transactions in blocks with less than say 1% tx fee paid then the rest of us would be pretty screwed, but then if they did that they would be disinceting the use of BTC which would cause the price to fall and them to make less money mining.  Doesn't make much sense.

There is a simple way ASICminer could gain a 51% position, but demonstrate to the network that they are not attacking.  All the have to do is spread their hash power out over several of the larger pools.  If they did this then they would need the cooperation of a majority of the pool proprietors to attack the network (already a possibility and not perceived to be a realistic issue).  If it were me I and I had 51% of the network then I would put 1/3rd of that power each on deepbit, slush, and btcguild.  This way none of those operators would go over 50 (they would each be closer to 33%), and I would not actually be over 50 in practice because I would only be computing the hashes they gave me.

Or not if you are into anonymity.

Practically forever

I have a long involvement with the community.  I have signed the following message with a btc address which has been active for several months.

Address: 1PwCJmiXzwUM1ixJx5ho1XFZzb6oh5W2ki
Message: My bitcointalk.org username is pwrgeek
Signature: H0PouHbwU9u2grYcFyu88ci1i5FhNRoACmFQCgT2x5jmcrp3xocXrGCTVvGm1hUZkxNZduaRlHn0h+kZ2IRs4Xk=

http://blockchain.info/address/1PwCJmiXzwUM1ixJx5ho1XFZzb6oh5W2ki

Hello

Seems like you could also have a system of reputation.  For instance I'm new here, but have been active for a long time on other Bitcoin related sites.  Could probably get someone to endorse me.