In my opinion, which I could be completely wrong and off base. I have a feeling the current iPhone 4 design is here to stay for 1 more year. I think it's a 2 year design. Since the Verizon iPhone has a newer antenna scheme and the chip is capable of cdma and gsm, we might see this phone or gsm only with the updated antenna scheme on Att. I think we might also see the elusive white iPhone 4 as well. Since BestBuy, ATT, and a couple other stores are seeing white iPhone SKU's in their databases, this could be all we see.
Maybe they'll update or tweak a couple things, but I don't think we'll see iPhone 5 this soon. The iPhone 4 is still extremely powerful and it's still out performing many phones that are just now coming out. I think the revision will just be like the iPhone 3g to 3gs, not a whole lot was changed.
I'd say get it, what do you have to lose? If in fact they do come out with another upgraded model, you will be able to sell this one for pretty much retail price and break even on the new one. That is, if you take care of it 😉 .