I set the option name to 480x270 because that was the resolution of the fmt=18 stream provided by YouTube. That still applies for older videos, but YouTube have now increased the resolution of that stream to 640x360. They did this a while ago, so any videos uploaded after the change will be displayed in 640x360. For these videos, there is no way to obtain a 480x270 stream. The quality options available for download and playback will be as before, though I've changed the name of the 480x270 option to 'High quality' to take account of the change.

I'm not sure what exactly causes some videos not to play in the existing version (no errors are reported), but as you say, it seems to be a buffering issue that causes the player to give up. I decided that video playback had too many problems (sluggish UI and rotation bugs), and the time would be better spent re-writing the application from scratch, as there were too many things that I was unhappy with. So far, I can say that signing in, loading search results etc is faster than before, because the code is leaner. Hopefully, I can bring similar improvements to video playback.

I think a decent 3g connection should be more than capable of streaming 640x360 video in most conditions. I can do it on my Orange San Francisco using the Android YouTube client, so I see no reason why it can't be done on the N900 in Qt.