Lots of people are asking about WordPress.com, fortunately for them there has been lots said about it already, both on the wp-hackers list, and on the WPMU blog. Usually without much feedback which I have found strange, but there you go!
The question comes up on IRC frequently, and here’s what I said a short while ago:
<donncha> it’s just WordPress.. if you already run a WordPress blog then you know a lot about what WordPress.com will be
It’s that, but also more. WordPress.com is going to be a great site and a shining example of what can be built on GPL software!