Definitely wait for this summer, especially if you can go month-to-month until then. Hopefully the huge jump in other smartphone like android combined with the slowed growth of iPhone sales will result in Apple making some big changes.
I've been eligible for an upgrade for a while now, but I'm definitely holding off until this summer for the fourth generation iPhone. If Apple doesn't deliver something awesome I'm considering a jump to an Android device.
Ditto on the possible switch. I love the iPhone and have purchased some apps, but not enough to make me stay with it if a competing product looks better. Besides, I can use those purchases on my soon-to-be delivered iPad.
To be honest, in light of the new devices out there the iPhone basic interface is looking pretty dated. The iPhone hasn't been out too long, but in terms of handsets it feels like a lot longer. I think Apple needs to at least refresh the look of the homescreen and incorporate some widgets that periodically update when other apps aren't being used. Also, I have to agree that some multi-tasking and display customization would be nice. Also, AMOLED screen and a good power boost would keep me with Apple another couple of years.
Let's hope that, despite the Nexus One's lackluster sales, Apple is feeling the pinch of competition enough to make some significant improvements over the typical marginal changes that are just enough to keep sales going.