UPDATED: 2009-10-19
I'm not sure for how long this little tip will work. I figure when Akamai & Apple catch on to the fact that people are watching a stream they intended for the iPhone, they'll put a stop to it.
But in the meantime, you can watch a very high quality stream of NASA TV in Snow Leopard's QuickTime X Player. Just launch QuickTime Player, go to the File menu and choose “Open URL…”. Then enter the following URI:


Old URI: http://qthttp.akamai.com.edgesuite.net/iphone_source/yahoo/nasa/nasa_all.m3u8

You can also follow along with STS-128 with MissionClock for iPhone and iPod Touch.
I've been watching for the past hour or so, and it's not without its problems. It's about 45 seconds behind Spacevidcast, which seems to be the lowest-latency stream available. Every now and again it will stop momentarily. Sometimes the window shrinks and turns black for a while, then comes back. But overall, it's a high-quality image that should work on a variety of connections, and doesn't suck down CPU like watching in a Flash player. However, it does cause the fans in my Aluminum MacBook Pro 2.5 GHz Core2Duo to run a little bit, and I suspect Apple could do some work to improve this.