From a hardware perspective, now that they have the iPad and iPhone versions of iOS synced up, I wouldn't be surprised if they had a fall release of version 5. As the iPhone 5's hardware is probably nearly identical to the iPad 2's, it wouldn't be hard to release it with iOS 4 and a promised upgrade as was done with the original iPad.

Add into this that they're probably in crunch time to get Lion out the door, and a fall release seems even more likely.