I'm also having this problem just as of today in the right sidebar. I have the javascript code for my ad sponsor (Six Apart Media/Adify) loaded into a text widget at the top of the right sidebar - but it won't show up. I have no other ads displaying on my site.
This is new as of this morning, it had worked for at least a month prior. Any clue?
Thanks much for any assistance.
www.the-craftycafe.com
ETA: It's working now but thanks much.