The worst thing I could possibly think of happening is that every app that thinks it has valuable information to show starts adding a widget to my screen, so (even as a developer myself) I really hope Apple do NOT give a full widget API. Instead open up a completely separate store just for Widgets just like we have for Mac OSX.
I personally find widgets to be rather annoying anyway and as someone previously pointed out the new notification center messages can actually handle important data far better than widgets. I removed the stocks widget within 2 minutes of trying iOS5 and, although it's a novelty right now, i'm sure the weather widget will be removed from my iPhone in the near future.
That's just my personal feelings on the matter!