I have wathced Lost from abc.com using Safari on my Mac. It did require me to download and install the SilverLight plug-in. I do not know the situation for other OS and browsers.
The ABC site (the "abc full episode player") isn't even silver light, it's some kind of half-assed, totally custom plugin. From the site (when browsing on linux):
Oops
Our new video player is only available for:
* Windows XP/Vista - Internet Explorer, Firefox
* Mac - Firefox, Safari.
To watch, please download the appropriate browser.
You can just run FF in wine and watch it from there. It works for me. Sometimes the video doesn't appear, but if you hit full screen then return it to normal, it starts working.
Because I really like J.J. Abrams, the writers, and many of the actors on Lost, and in some small, small way I want my act of watching their show to reflect positively on their compensation.