With LR 4.1 RC, the Flickr publisher seems to be broken. I added 2 new photos to a collection today after updating to the 4.1 RC, and tried to publish, but I get an error message: "Can't update this collection. An internal error has occurred: Invalid at the top level of the document."
Publishing the same collection (without the 2 new images) worked fine yesterday in 4.0 before the RC update. Closing and re-starting LR did not solve the issue. Running Vista x64 SP2 with all updates and 64-bit LR installed.