That's interesting. Which part of the Knol TOS specifically, is it #8: http://knol.google.com/k/-/-/si57lahl1w25/12# which grants Google the right to "modify" and "create derivative works based on" what you submit to them? Why is that incompatible, is it because this grants Google the right to "create derivative works" which take out the sharealike requirement, and you're not allowed to do that?

Yes, it's section 8. It's not anything unique to GFDL, you can't upload anything to Knol that you didn't write yourself (or you have explicit permission of the author) since you can't grant Google a license to something you don't hold the copyright for. It's worth noting that uploading someone else's work wouldn't be a copyright violation because of that, it would just be a violation of Google's terms of service, and Google would be committing a copyright violation if they tried to use the license you were meant to grant them but weren't legally able to.

(Again, IANAL, YMMV, etc., etc.)