Ben outlines the reasons for his concerns and I've read them, but I still don't know enough about the ins-and-outs / pros vs. cons on the issue to decide whether this is a bad idea or not.
That said, as an 'ex-pat', living in Seattle, I do miss the live / streaming content from the BBC site (example: the World Cup commentary here is...unfortunate...).
If my understanding is correct, having a commercialized (z) version of the BBC site for viewers outside the UK would mean getting all the streaming / live media goodness too. Currently, this is blocked from me as my IP address is outside the allowed range. It may be short-sighted of me, but for this reason alone, I would probably support the move to an ad-supported service. I thought this a while ago.
Am open to be persuaded on this either way though.