Here is the case, a direct competitor is intentionally bidding on a misspelling of your trade mark to direct people to their website. Lets say your trademark is raatsnacks (2 As) and someone is bidding on google for ratsnacks (1 A) and is a direct competitor. Does anyone have any links to any cases like this?
Seems like this link would have the information you need: http://www.google.com/tm_complaint_adwords.html It was found simply searching adwords and trademark.
This is unlawful if and only if it can be proven that the name used by your concurrent can evoke your brand in the mind of the public. Useless to say, there's nothing you can do here as you would need to go to court and it would cost you much more than what you are losing now, except if you're a big potato.