I had that problem too, although it only happened when my mac was signed into .mac (in system preferences). I tried everything i could think of like repairing permissions and deleting preferences, but eventurally i was mad so i did an archive and install.
No doubt you should see if anyone has any bright ideas first though.
Try the Apple iWeb support page as I read earlier today a bunch of people had this problem as well as other problems and somewhere on the support pages, someone found a work around and has it posted (can't remember just where).
Wish I could help more but I haven't upgraded yet.