My understanding is that they haven't sold any ads yet. At present, they are injecting "Chumby 101" tip widgets and public service announcement video widgets into users' custom widget channels. I believe they are doing this to get us used to the fact that we'll be seeing stuff we didn't choose or necessarily want in our channels.
Right now - maybe forever - the Chumby business model is to sell the hardware with very little markup, with the idea of generating revenue over time through sales of ads. Pretty much every question about ads that's been posted here has been officially answered "we haven't decided yet." So we don't know yet whether we'll have the ability to opt out by paying some monthly fee, how frequently ads will appear, what they'll be like, etc.
If the idea of ads on your Chumby bothers you, I suggest you wait for them to decide how they're going to handle them before making your purchase. Like Ed S, I didn't realize they'd be injecting unwanted content into the channels when I ordered mine. In fairness, that info is and was on the Chumby site, but it's sort of inobvious and vague. This issue has been raised many times; the Chumbians have said they're going update the site to make it more clear in the future.