Aha. On further investigation, it seems that recent versions of the Tor Browser Bundle on Windows and Mac use port 9150 instead of 9050 for some reason, causing every application that tries to use Tor (except for TBB itself) to break completely.

First of all, do not enter 9150 as your SOCKS Port in Bitcoin-Qt. Although this will appear to work, Bitcoin-Qt will not realise you are using Tor (since you're using a non-standard port) and will accept non-Tor connections, potentially compromising your anonymity.

Instead, you should add SocksPort 9050 (on a new line) to your torrc file. Leave ControlPort and the existing SocksPort line alone. eg:Do not do this from within Vidalia, as due to a (probably) unrelated bug Vidalia will not save the file correctly if you have multiple SocksPort lines, instead edit torrc in a text editor (be sure not to save it torrc.txt, as Windows tends to "helpfully" do), then restart Tor.