I think I've figured out why I wasn't seeing 'ads', and thought I'd share for three reasons - 1) in case people what to use this as a strategy to *not* see this extra content, 2) in case someone else was in my position and *wanted* to see the ads but wasn't getting them, and 3) in case Chumby see this as a 'loophole' and want to plug it.
From the start I've organised my channels to have only a few widgets in, and made multiple channels instead of packing lots of widgets into a few channels. It seems that this is what was denying the 'ads'. I'm guessing that every time the Chumby has shown all the widgets in a channel it resets the internal counter that's totting up to see when it can drop an ad in. So a channel with say, 6 widgets in, will never reach magic number (10?ish) to have the extra app pop up.
So here's my advice (based on how Chumby behaves now... but who knows, Chumby Industries may change this...)
- if you want ads, have 10+ widgets in a channel
- if you don't want ads, have fewer (6?ish) widgets in each channel