Using Qt and webkit.  It's been done a few times.  I even run a special page for one of my infocasts that is like a mini control panel.  I have plans on integrating it into home automation, alarm, and more.  Right now it only grabs the weather, some google calendar data, and cron runs my sprinklers.

Do a few google/forum searches on "chumby qt" as well as "chumby webkit" and that should get you started.  If you need help cross compiling or anything, post specific questions.