If you have an ASIC for Bitcoin mining, can you use this also for certain alt coins?

Trading I would say.

Yes, there is. Google something like: bitcoin rent hashing

No, you are wrong. Read: https://en.bitcoin.it/wiki/Difficulty

No, it will be around that date. Every 10 days the difficulty is adjusted to the hashing power, so that a block will be mined every 10 minutes.

Because it is hard to set up an exchange, I imagine. And with already having an exchange in place (with Bitcoin), you can easily use it as an medium to trade Euros for alt coins.

- When Bitcoin fails.
- Only thing I could think of is security.
- Off the shelf solutions for protocols that need security.

Yep, nowadays it's about 5W/Ghash.

Well, the Bitcoin network does not waste a lot of energy. It is in direct proportion to the security of the network and thus usefull, and inherent to the protocol.
I very much like the analogy described on this site:
http://expectedpayoff.com/blog/2013/03/22/bitcoin-and-the-byzantine-generals-problem/
It shows that you must do some kind of work to make sure the network is not flooded with messages.
That is the price to pay for having a secure decentralized currency.
Centralized currency also have to secure their use in a different way, which is also wasting energy. So essentially it is a trade-off.
When you add all the power used by the network it seems a lot, but just look at what it costs to power the LHC. You have to put things in perspective.

Best way to get a lot of traffic is to give away free Bitcoins, like in a Bitcoin faucet.

You could by an ASIC, but be aware that you most likely never get your investment back. It's expensive nowadays to mine Bitcoins.
Not to throw you off, but you are always welcome to mine for increasing network hashing power for a better security.

Can people who are mining alt coins tell how much you can earn with that?
Would be nice to have some kind of list which states how much you can earn as function of hashing power.
I am mining Bitcoins myself and can mine 0.0015BTC per day with 5Ghash.
That's about 1 euro per day.

You can also check sites with a collection of sites which use Bitcoin.
For example: http://topbitcoinsites.com/ and http://betwithbtc.com/
I think the last one also reports if a site is a scam.

Reading all the stories it sounds real easy to get a quick profit with mining. But I believe these days are long gone.
I bought my first 5Ghash asic in march this year. I got it finally in October and since then I barely got my investment back.
Now I mine 50 Euro cents per day, and that's already minus the electricity costs.
With the prospect of a rising difficulty and many more miners, I wouldn't recommend mining.

But if you just want to contribute to the safety of Bitcoin by adding mining power, I can only welcome that.

BTW, you can probably make more money by trading Bitcoin.