Can't Destroy any block in Minecraft 1.6 Prerelease using a sword while playing on Creative Mode, I have been having this problems since the 1.6 snapshots. I Also got a Video in Youtube Showing it:
https://www.youtube.com/watch?v=CDGYpeGmDiI
READ: This video was made using 13w25a but it still happens while I Play Minecraft Pre-Release 1.6 and it also happens when destroying any block other than sand