Forcing YouTube to Play HD Videos
Normal Mode and Hi-Res Mode
Provided that the user, you're watching, uploaded a high quality video with the specs above, you should be able to force YouTube to display hi-def videos even though you're not logged into YouTube.

For instance, this video displays in normal mode:
youtube.com/watch?v=PmqQKZPlj6M
Now, for the same video to display in hi-res mode, the trick is to append &fmt=6 or &fmt=18 to the end of the URL.

&fmt=6 increases the resolution from 320x240 to 448x336, Flash 7 video @ 900Kbps; audio @ 44.1KHz 96Kbps Mono CBR.
youtube.com/watch?v=PmqQKZPlj6M&fmt=6

&fmt=18 increases the resolution to 480x360, H.264 video @ 512Kbps; audio @ 44.1KHz 128Kbps Stereo.
youtube.com/watch?v=PmqQKZPlj6M&fmt=18