nudge:
Yeah I think so. I am not completely down with the rules, but I brought all my apps (including TomTom) a while ago, on my 3g and synced etc.
Then got the iPhone4 and synced all the same apps to the new phone. Not a prob. In fact I use my laptop to sync to my phone with my music etc and the Mrs got the old 3g on the desktop, syncing the same apps and different music.
I think the key is to use the same Itunes account. A mate told me this is actually allowed as Apple have a 2 phone policy thing.
Thanks :) that will make it alot cheaper!