What do I have to take into account if I want to replace my iPod touch with an iPhone?
I've not tried this myself but this guide on ehow.com makes it look fairly simple using iTunes.

Basically as long as the host computer is authorised for iTunes use, you can connect your old iPod, right click on the icon from within iTunes and select Transfer Purchases to get the apps onto iTunes. Then connect the target device and sync the applications to it.