With the de-centralized approach of git, this is no big problem, you can commit changes locally and merge things back globally at a later time.
I have by now made some nice experience with collaborators all over the globe using this...
The scribble tool can be found under the shape button. Stingy as I am I've only used free online whiteboards so I really can't complain.
The best of these I've found were twiddla, scriblink and skrbl.
I imagine that makes it feel more like writing on paper, but I'm waiting for those to merge with computer tablets.
Personally, I prefer git over other more centralized solutions like subversion.
It has several nice advantages when you're using different computers (say a desktop in your office and a laptop on the train or so) for which you do not have always a reliable internet connection.
The most annoying part as I recall were the online whiteboards which could crash or behave strange, not have enough writing area or didn't allow saving the content.
Correction: Google docs drawings may be used as an online whiteboard and allows one to save the content.