Before you even get to the Marketplace blade, the Xbox Live blade was an entire upsell blade if you weren't paying for Live. Depending on your opinion of "ads", that counts for some people as a giant ad. (I don't entirely agree with that sentiment myself, but it is something I've been asked to train myself to see.)

(ETA: In a similar vein, the bright logo of Windows Media Center in the Media tab would look like an ad for an upsell to many.)