If this is a new concept to you, imagine:
When I first received my BFL Jalapeno (ASIC 5G mining rig) and started mining, to the time I sold it, the difficulty had increased enough that I was seeing a .05 BTC gain in twice the time. So now you can imagine how eventually only people above a certain mining speed will be profitable.
Yes you can. It's called pooled mining.
http://www.bitcoinx.com/profit/
At the next difficulty estimate, an i7 with free electricity mining all the time at 8 MH/s would make about 9 US cents in a month. Since the difficulty will probably go up again in less than two weeks, make that less than 2 US cents per week. Since very few people actually have free electricity it's not worth it.
Seriously, you can. Just based on your comment alone:
> an i7 with free electricity mining all the time at 8 MH/s would make about 9 US cents in a month
This means at the current rate you will make
0.09/470 = 0.000191489362 BTC = 19148.9362 satoshi
Whether you can actually get a single satoshi from a pool is another story. If the pool uses PPLNS, you might not ever. I think most would object to you for being misleading. If you want bitcoins, buy them, or get an ASIC miner and don't use a sucky-ass CPU or GPU.
I specifically addressed electricity costs in my original comment.
My point was, you can make BTC even with a shitty CPU. Whether it's worth it is a different question.