I had a Storm as well. When you unlocked it, you could get it to work on AT&T and T-Mobile, because it was a global phone with a SIM card. You couldn't get 3G on any of the US Carriers, since the 3G frequencies are different in the US and in the rest of the world (so the data would be confined to slow 2G rates).
I did see a rumor recently that T-Mobile eventually did something to lock US Storms out of their networks.
If you get the Droid 2 Global, you should be able to unlock it and run it on AT&T or T-Mobile with a SIM card. You most likely won't be able to get 3G, though. Probably not worth it.
Hopefully, when Verizon switches to LTE some time in the next year, the phones will be compatible across carriers. From what I understand, Verizon, T-Mobile, and AT&T are all switching to LTE. It sounds like Sprint is investigating, and my guess is that they will go that direction (ditching WiMax in the long-term). However, I wouldn't put it past the carriers to lock other US devices out of their networks.
Regardless, this won't help much with getting a device to run on Sprint. I'm sure that there will be some more cool Android Devices coming out for Sprint in the future.