Here is how you hack YouTube to default into HQ:
1. Put the youtube.apk on your sdcard.
2. Get Root Explorer from the Market.
3. Using Root Explorer, navigate to the sdcard folder and long press on the youtube.apk and click "Move"
4. Navigate to the /system/app folder
5. Click the button up top that says "Remount as R/W"
6. Click the button on the bottom that says "Paste" then confirm overwrite.
7. Now you need to set the permissions on that file: Long press on the youtube.apk and click Permissions. Check User Read and Write, Group Read, and Others Read. Uncheck everything else and click Ok.
7. Click the button up top that says "Remount as R/O"
8. Reboot, you're done