Very clever Google.
I use GMail as my primary mail client and I can tell you I don't think I've clicked on an ad, or even noticed an ad for as long as I can recall. When you use an app like GMail, advertising is totally ineffectual because you just develop ad blindness as your eyes are trained to go where they need to go.
My eyes are trained on the other hand to quickly flick up to my Web Clips (that's the RSS posts that Google throws up onto your front page) in the hope that one of my favourite blogs has a new post that I might click over to.
Just now, however, for the first time that I've seen, I got fed a "sponsored link".
It did a pretty good job on the relevance side of things feeding me an ad for "Writing Opportunities. FreelanceWorkExchange.com – Freelance jobs for Writers…"
Yes, it annoyed me. I don't really want my RSS feeds punctuated with ads but it's not going to annoy me enough to persuade me to ditch GMail if its not done with too much frequency.
I do wonder how they're working out the relevence based on my Inbox though. If they're only using the text from my message headings that's surely going to throw up a ton of irrelevent ads. For example, based on my message headings you'd probably be serving me up lots of ads about squash equipment.
Whatever, the case Google has to solve the ad blindness problem and intermixing ads with Web Clips is a good way of doing it and they'd probably do well to extend the same idea to the right-hand column.