Landyman, i've also got a MacBook, do u use MissingSync? If so, u can make a backup of it with that - however, Missing Sync does not work on Windows Mobile 6, because u need activesync 4.5 for it to work.
I have the same problem.
I backup my stuff via Outlook under Parralels, alternatively you can install BootCamp to emulate Windows XP, then install Outlook there and sync with that via ActiveSync 4.5
I spoke to MissingSync's guys and they said they do not support ActiveSync 4.5 / WM6 yet (obviously cause it's not officially released yet)
So in short... you're gonna lose all your notes and Office Document files etc etc on your phone when you do a sync back to your phone after you've upgraded your ROM.
It's a shame tho, cause the Black 2.5 ROM is pretty nice looking and also has a nice backup utility built-in...