Actually I am not just worried about annoying the end user of gmail, I am also worried about the non-gmail person writing to them. I appreciate that Google is not charity and needs to finance the service. However,in someways what Google are doing doesn't make business sense and may irritate some users.
Firstly, it could be a sensitive mail and it may appear to distasteful to the reader to "cannibalize" it for advertizing fodder (how will the technology determine whether it is inappropriate?). People don't like to have things "thrown in their face" when reading an email, which is why I suggested a button that they could press. (And lets face it, if somebody presses a "Google suggests" button, they are more likely to make a purchase than if the advertizing was just foisted upon them (albeit with their consent)).
If I write you a long rambling mail (which would be par for the course) how will they know which paragraph contains references to something I would actually want to search for? I might send you a long joke and have no interest in searching for anything related to the joke. Alternatively I might spend 3 paragraphs droning on about the weather, but the really meaty stuff could be in paragraph 4. So I'll see 3 ads on the weather and 1 on what I really want to search for. It all appears pretty hit and miss to me. In addition to the suggestion button, they could have something like a right-click menu, which, when you highlight a phrase , a new window opens with the search results. Personally I would prefer this to be organic serps but I can understand why Google might just want to show ads in this new window. Either way it would use the adsense technology to ascertain the context of the search term from the surrounding paragraph(as opposed to just using the term itself as it would with a normal Google search). This would skew the results in the direction the user wanted rather than trying to secondguess them.
Just a thought
With Kindest Regards
Please Be Gentle