Hi Mark,

It is question that I have asked many times and the best answer that I have received is the following:
The domain name should include some important keywords (i.e if you were trying to sell nokia 8800 mobile phones then you would have www.nokia8800.com or nokia-mobilephones.com).

This is the ideal situation to be in, although you can have success with such domains as :
www.mobile-phones101.com/nokia8800.html

The key is to have the targeted keywords displayed in your link (domian name) and your title of the page. These are currently the two most important parts of a single webpage (plus original content!!)

Hope this helps...good luck.

Cheers
Chris Wainwright