This is just an opinion based on experimentation with both hyphenated and non-hyphenated domain names.

Google doesn't care whether a domain name is hyphenated or not, provided that the keywords are there. For example web-site-name.com and websitename.com will produce similar results.

MSN, however, absolutely loves hyphenated domain names, at this time anyway. That may change.

I suggest you take this concept one step further though, name web pages using logical naming conventions as well. For example, a web page named web-site-ptomotion.htm is much better than one named bunchoflettersandnumbers.HTML

Do this with graphics too, especially if you want your images to be searchable. Same theory, web-site-promotion.gif is way better than buchoflettersandnumbers.jpg . Throw in an alt attribute "Web Site Promotion". Provided that this technique is used sparingly, it can be effective in gaining additional weight for that page for the keyphrase.