In my limited experience Mat is right with google - you can have hundreds of ads/keywords pointing to the same page.
The keyword does not even need to appear relevant.
However, with overture & mirago you can still have many keywords pointing to the same page but the keywords must appear to be relevant - this can be a real pain because what appears relevant to one person may not do to others.
eg run a campaign to sell widgets. Blue widgets are more expensive to bid on than Red widgets. Your landing page makes it clear you are trying to sell Blue widgets but at the bottom you have a small piece about other colours of widgets - may even mention Red widgets - with a link out .
Overture/Mirago may view the page as relevant only to Blue Widgets & thus reject your Red widget keyword
Maybe not a good example
