Makes me wonder if these are new 3G users on T-Mobile's network. Switching between 3G and Edge is quite common, and entirely a function of the 3G coverage in your area, which is often spotty, but slowly improving. I see this all the time on my G1, but it happens less and less as 3G coverage improves. The issue seems to be that the towers that support 3G have a different range than the towers that support GPRS. Naturally, T-Mobile tries to do the cost-effective thing and only upgrade the towers that will give them the requisite coverage. However, that isn't always easy to predict.