I am aware that themes have changed a lot in 2.0 and since RC1 is out I would like to start converting some of my themes.
Is there any guide / list of differences in 2.0 vs 1.1.x theming, or do I have to chew my way through 2.0 RC1 default theme and try to figure it out?
Start chewing. :)
The good news is that obviously you only need to do the files contained in your custom themes, and once you've done one theme the rest shouldn't be too bad.
I doubt anyone will be writing a conversion guide, simple because there are a LOT of changes. Running the upgrader with your theme installed should update the changed strings for you, however you'll still need to add bits of 2.0 functionality