Make it sure your HD videos are mp4 3GPP, MOV, AVI, MPEG2, WMV,FLV formats, and then log in to Youtube, then to upload your videos to Youtube, then you can play it on Youtube.
The YouTube app will automatically play the HQ version when on wifi. It will play the low res video when on 3G. I believe the workaround is to jailbreak the device and use a third party app. I've been told by others I am wrong. **shrugs**