I've been looking into the same question myself for the past few days, and was going to make a thread as well. It's indeed a very good question that should be stickied in some form at the top of this section for all new people to read, as I don't believe enough people actually take the time to figure it out before dropping pretty big bucks on them.
As mentioned earlier the profitability calculators are a good way to figure this out:
http://mining.thegenesisblock.com/After trying many different things in the calculators I believe I can safely say no, you'll never see a ROI on these even at the new "low" prices. Difficulty is just increasing too fast. At 50 bucks for one you'll end up paying $12 to run it for 6 months at which point it'll consume more in power than it makes. I feel sorry for the poor saps who just a few days ago payed .90
BTC/$80 for them. At that price, if they bought 5 they'll pay like 200 bucks to run them for 6 months. Basically they're paying to increase the network hash rate and drive up difficulty even more, therefore making even less ROI.
The way these things sell in group buys you'd think they're magic money machines, and I was almost ready to drop $300+ into BTC to purchase some of these before I actually did the math and used the calculators to help, so I can see how easy it is for new people to get swept up in this thinking they're going to profit. The only ones really profiting at these prices are the manufacturers selling them.
To actually be profitable they need to come down into the $30-35 range, or increase their has rate quite a bit. Doubling would probably do it. Even at $30 each on something like the K1 nano you'd only make like 20 bucks running 5 of them. Maybe 50 if you could manage a pretty good overclock, but difficulty increases will likely kill that as well if you're starting in Aug.
I believe I'll just hold out myself and see what happens. If they drop to a lower price where I could even make 20 I'd probably pick up a few just to have fun, but I'm certainly not going to pay to run them.