There's still some features and things missing from the 64-bit builds for Windows, handheld support being one of them. Not sure about mms:// streams though, maybe Jim/Matt/Hendrik knows?
Likewise if you're using an AISO driver for audio output, make sure there's a 64-bit version of the ASIO driver available from your sound card/DAC manufacturer otherwise it likely won't work with the 64-bit Windows build.