EDIT: Here's a chart that claims to show the top spenders on AdWords, and there aren't any search engines I can see:
Btw, I don't think it makes sense for Bing to buy the keyword search engine. I can't see how that makes any sense. Unless consumers are REALLLLY dumb. Then again, I might be expecting too much.
That second link pre-dates Bing's launch, I believe. You'll see Yahoo and Ask.com in the top 15. Much of the top 20 are product, travel or local search engines.