Forum Moderators: martinibuster
Now that my sarcasm has almost run it's course, the closest Google comments on this is:
No Google ad or Google search box may be displayed on any domain parking websites, pop-ups, pop-unders, or in any email.
Technically a forum post is a form of message but not necessarily "email". You are limited to 3 ad blocks per page so if you put it in your signature you could violate that without too much trouble if you post too many replies to a thread.
Personally, I would write to Google support and see what they say as this is just way to fuzzy gray of an area but I'd be shocked if they said it was ok.