What will it be like? That depends on whether you jump for the iPhone 5 expected to come out this fall or the iPhone 6 expected to come out in 2012.
If it's an iPhone 5:
1. Expect it to be thinner and lighter with an eight-megapixel camera on board. Other than that, it's pretty much a retread of the iPhone 4.
2. It should be available towards the end of September, barring production delays. There's word that the iPhone 5 is so thin, manufacturers are having a hard time installing the components.
If its an iPhone 6:
1. You definetly won't get it in time for Christmas. It's not due out until sometime in 2012. It may be worth the wait to sit out on the iPhone 5 cycle, however. The iPhone 6 is expected to feature wireless recharging (that would be big!).
Regardles of which you upgrade to:
1. Sooner than later, you will likely have at least three carriers to choose from. There is fairly serious speculation that Sprint will be next to offer an iPhone. This would be especially attractive to both Apple and it's users, since Sprint is now the only major carrier in the United States not setting billing limits on monthly data use.
2. There is even more continued speculation that Apple is working on offering a smaller, more basic model of the iPhone at cheaper prices for those with less jingle in their pockets.
Your next iPhone may be:
1. An Android. Why wouldn't it be? All the major carriers are offering them. Choices and pricing are much more competitive. Samsung is going crazy offering 2-D and 3-D built-in HD recording cameras. Next up, flipping between your smartphone screen and an eInk screen?
2. A Blackberry. But, I doubt it.
3. A Windows Phone 7. But I really, really doubt that one.