The unmetered data applies to the CASPA content only - ie if you downloaded a movie off itunes for instance, you'd pay for the movie and the data used would chew into your cap. So Revision would use your data. I can't see how the Tivo box would access other sites directly so anything other than CASPA stuff would need to come down the pipes to your PC first and then be streamed from there to the Tivo.
The only un-metered content is content the is sourced from CASPA, anything else pulled across the wire is chargeable.
As far as other applications to do web streaming I'd recommend having a search for TiVO HME applications on google and see what you come up with. I haven't played around with a lot of them yet, but it's worth a nudge if you're interested.
At the moment the NZ/AU Tivo's can't played "streamed" videos from streambaby (and as you said have no Youtube capability built in) for example as no one has worked it out yet.. I would try pretty hard as I've had dealings with streaming video for quite a while.. but alas.. $1000 for "testing" is a little too much for me at the moment.