The 10D has a few more features and a heavier duty casing. I got one of the 300Ds about 3 weeks or so ago though and can't be happier with it. God it's an awesome camera!!! I was coming from an old Sony and then a Minolta DiMage 7i, and even as nice as the 7i was, there's literally no comparison between it and the Canon. http://www.thephotoforum.com/forum/viewtopic.php?t=2359 is a thread with some recent shots from the Canon.
The rebel digital 300D is a great camera for its price range. At first glance, you would say why would anyone consider paying more for the 10D? However there are significant differences. For a quick overview, check out this neat comparison charts I found:
The review as a whole is very good and will help you make a choice. You have to decide if all the extra functions necessary and are you willing to pay the extra $$$. I am new to digital and did not want to spend 1000+ on a digital slr ( $899 for body + compact flash cards + any extras like another battery). Ever since the rebel came out, prices on used models have dropped. I just picked up a used 30D with all the fixins relatively cheap that has me satisfied so far ( only had it for a short time ). Yes, 30d has half the megapixel but it fits my entry level needs and then some.