If you've just created a YouTube account, you may have noticed that the videos you've uploaded can't be viewed in High Definition yet.  This will change after you've had the account for a while, uploaded a certain number of videos, and or after you get a certain number of views.

YouTube just enabled the videos on my account to be viewed in high Definition and I currently have 52 videos, 2347 total views and my account is 6 months old:

Eventually you will see this message when you upload a video:

Not only can viewers view the videos in high Definition now, but you will also be able to upload videos longer then 15 minutes and in a wider variety of formats.