We've discussed it a lot with regard to Apple TV, GoDuke.com Inside Access, and a general streaming games online thread.
Bottom line seems to be that whatever your TV/ISP company lets you stream online, that's all you can get legally. As of now, ESPN only works through cable, satellite, and ISPs, not directly with customers.
There are plenty of illegal sites, which are a landmine of malware/crapware/ickware, but if you're careful, you can watch them without downloading unwanted extras.
See the threads above for some good background.