I think it will be bigger leap than you think.
If it is just incremental, they would be on time for another upgrade for June.
The next hardware upgrades as far as processing power are obvious. Economies of scale dictate they are going to use their latest processors across as many product lines as possible (the next iPhone and iPod Touch are as good as guaranteed to have an A5). I think a delay would more likely be attributed to iOS 5, particularly since it doesn't look like there will be a big iOS 5 unveiling at WWDC. I actually hope this is the case. It would indicate significant changes to iOS, and it needs it!
EDIT: As a side note, fingers crossed they bring Core Image support in this version. Graphics hardware is up to the task now.