Just rebuilt an existing app we had into a fat version, with iPad res version and regular version in one.
Total win.
Simply duplicated the master scene, changed guitextures etc, the majority of the scripting was totally intact. In a few places I added a ‘isPad’ boolean to handle things that were hard-coded (scrub-areas for text scrolling etc).
I found the build documentation a little lacking, but overall, I’m extremely happy with how smoothly it all went.
Thumbs up to the Unity guys 
Things I learnt
- Hardcoding screen locations is bad
- Using transforms on GUI Texture’s is smart, rather than pixel offsets (although pixel offsets are far more intuitive when you’re working in a locked res)
- The iPad fillrate is a joke (i knew this, but I hadn’t experienced it for myself)
- Your desk looks awesome when you’ve got 2 or 3 ipads, a few ipods and iphones in front of a few 24" monitors. Makes you feel tough.
- Full screen 24bit images DESTROY MEMORY. You might have double the ram, but your images about about 5x larger, so you end up behind… Unfortunately, we didn’t choose a colour palette that dithered too well into 16bit, so some nasty workarounds had to be made.