There's a useful profitability calculator available here:
http://www.bitcoinx.com/profit/Unfortunately, in the site's own words, "Extrapolating bitcoin difficulty or price is pure voodoo", and estimating return on investment requires an estimate of both. If all the ASIC vendors promising delivery "real soon now" actually do deliver, the current total network hashing power will soon look pretty pedestrian, and the return from mining for a given level of power will fall sharply. For example, you can see a list of impending hashing power just for BFL in an entirely unofficial list here:
http://bfl.ptz.ro