I'm very undecided (and will be until Steve or Tim disclose details of the next iPhone). On the one hand, my 3GS is working OK, but I want a few of the improvements in the iP4: camera (vastly superior), signal strength, and some technological advancements over the 3GS. On the other hand, I don't like the glass back (only way to protect it is to put on a case that really changes the form factor).
If the next iPhone has HSPA+, a faster chip, better battery life, an improved antenna design and hopefully a carbon fiber (or non-glass) back - I will upgrade to the AT&T version. I don't know my 3GS will satisfy me long enough to wait until iP6. I'm too invested in apps to consider any change to Android (which I don't like as much anyway).