Though the percentage of Internet subscribers who will be able to view the video stream outweighs those who cannot, it was probably a bit inconsiderate of the PIC to chose Silverlight over more open and more easily accessible standards. This leaves out Linux, Mac PowerPC, and FreeBSD users, according to an upset open source advocate.
Though there is the Moonlight alternative for Linux users, it is still in a beta stage and won't be finalized for still several more weeks, and even then it will not be implementing many of the Silverlight 2.0 features. Moonlight 2.0 would be able to handle such features, but this will not be released until later this year-- a bit late for the inauguration events.
In the words of the Mono project chief, "I tried looking up the player on the Obama web site, and I guess the player has not been made public so I have no way of testing. The streaming and media codecs should be compatible with Moonlight 1.0, but the 'chrome' used to paint the player might be a 2.0 app. If people compile Moonlight from SVN, they can get our 2.0 support that might be enough to work with this, but we have not officially released that yet."
Since it's safe to say that most of the US population with Internet service who will watch the inaugural events via streaming online won't know any better to use a browser aside from Internet Explorer 7 or 8 on their cheap Dell machines bought at basement prices from Wal-Mart, there won't be too much of a problem for them. Still, it's the principle of the whole thing: why not include everybody and choose a more open and supported standard out there?