By default all videos on YouTube site and embedded on other sites load at normal quality, but there's a way to set your default viewing quality to high, link to high quality video, embed HQ video, and even save HQ videos for later viewing, Jason Kottke ran through a hack:
First, set your default viewing quality to high: just logged in to your YouTube, go to Account / Playback Setup / Video Playback Quality and set the option to "I have a fast connection. Always play higher-quality video when it's available."
Second, if you want to link to the high-quality version of a video? Just append "&fmt=18" or "&fmt=22" at the end of the URL. 18 is the 480x360 version, and 22 is the 720p version. Some videos will have one, but not the other, so try both if you need to.
When it comes to embedding, you can make a quick change to the embed code to get better video quality. Just add "&ap=%2526fmt%3D18" to the end of the URLs in your embed code. Here's an example:
<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/oHg5SJYRHA0&hl=en&fs=1&ap=%2526fmt%3D18"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/oHg5SJYRHA0&hl=en&fs=1&ap=%2526fmt%3D18" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>