@Gavin - BTW, I'm concerned with the transaction fee size itself, not the percentage (since I'd be on the hook for the fee, should the user not have enough BTC to cover the fee in their account).  And, it was my understanding that as the size of the transfer goes up, the txn fee does, too, right?

I added the functionality, and submitted a pull request: https://github.com/sassame/bitcoin/tree/estimatetxfee

Gavin, I understand your concern, but when thinking about the use case (specifically, checking during a transfer if there's enough bitcoin), there's probably a pretty low chance of the estimate changing.  I would still like to see the ability to add a "max tx fee" to the send rpc calls, which would address when the tx fee goes up after estimate.  But, this is better than nothing.

As a side note, this is the first C++ I've written in 5 years, I profusely apologize for it sucking.  I realize instantiating a transaction that you know is going to be destroyed is wasteful, but I didn't feel comfortable enough with the source to refactor the fee calculation code (since I'd end up touching many of the most important functions).  Do I need to do anything to dispose of the transaction?

There's actually a small issue with this accounting algorithm - if we don't know the transaction fee before hand, we can't actually ensure the user's account has a large enough balance to cover the transaction.  If their account has 0.25 BTC, and they try to transfer out the entire amount, the merchant would be risking the transaction fee when they executed the transaction.  At .25 BTC it's not a big deal, but as the sums go up the risk increases substantially.

I looked at the code, and it seems pretty easy to write an RPC call "estimatetxfee".  Before I get too deep into doing any development work, is this a realistic approach, or would the chance of the fee changing (especially for larger transactions) make this worthless?  Would it be possible to calculate a maximum possible transaction fee for a given amount taking into account the coins currently in the wallet?

Thanks for any help!