Having tried Litecoin CPU mining out of curiosity the other day, I'd not consider it worthwhile. My 2.0GHz i7 MBP was getting around 4 khash/sec (and running at 99C, so I bottled out). I then tried it on an otherwise pretty idle server, which got a steady 2.0 khash/sec.
Feeling smug that I'd be using spare, paid-for server cycles to make money, I looked up some expected earnings. (Warning: I'm an LTC noob, so may have plugged in bad assumptions), but even at 4khash/sec, I was looking at less than $3 a year
I'd like to be proven wrong on this, of course. However, talking to a couple of people on IRC (hardly guaranteed 100% reliable info, but good enough for me) it seems that even LTC has moved to GPU mining being a lot more realistic than CPU mining)
You might want to look into mining litecoin (LTC), which might still be profitable and whose mining is CPU-based.