Provided that the user who's video 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:
http://www.youtube.com/watch?v=MfWXRybZS4A
Now, for the same video to display in hi-res mode or HD mode, the trick is to append &fmt=6, &fmt=18 or &fmt=22 to the end of the URL.
&fmt=6 increases the resolution from 320×240 to 448×336, Flash 7 video @ 900Kbps; audio @ 44.1KHz 96Kbps Mono CBR.&fmt=18 increases the resolution to 480×360, H.264 video @ 512Kbps; audio @ 44.1KHz 128Kbps Stereo. &fmt=22 increases the resolution to 1280×720 (720p), H.264 video @ 1024Kbps; audio @ 44.1KHz 232Kbps Stereo.