Usually whenever I'm doing this, I'd just plop out a circuit in SPICESPICE and then print to a PDF. Not the most elegant solution, but not the most difficult either.
LTSpiceLTSpice is free and pretty great. Also, I hear they keep waveforms inside some of those circle and triangle things on the schematics,schematics; you just need to push some buttons to let them out :-)
Is this what you meant? Or did you mean something that is modifiable like a GoogleDocs for schematics? That'd be nice, wouldn't it?