i have been in the market for my first digital slr for a while, and just a few days ago i decided on the nikon d70s (used, $400). the next day, the alpha came out.
while i am very impressed by its potential, i do not regret my decision in the least.
the most compelling thing is the sensor, it is the same one (roughly) that is in the nikon d200 - everyones current camera du jour - but for half the price. lens look good, antishake in camera, etc.
all that said, it will be several years before the sony-KM system will be kink free and able to match the offerings of canon and nikon. don't get me wrong, in a few years, i am quite confident there will be three major players in the digital slr market, and sony might even be number 2 by then.
but dont go out and buy a system based on a single camera. buy a good lens or two, and the cheapest back available. learn how to use that camera until it is the camera that limits your photos, not your skill. then upgrade.
at this moment, imo, that means canon or nikon.