Ever since the web came along UI designers have had to change their tact from the desktop to the browser. Now the browsers are going onto every imaginable device. Websites are having to either produce many versions of themselves depending on the client, or produce one dumb-downed version that works on them all.

Here's a good article that discusses UI design issues for the devices of today and tomorrow: http://www.digital-web.com/articles/web_beyond_the_desktop/