To www or not to www?
One of the most important things you can do to improve the search engine optimization of your website is to solve the so-called "canonicalization" issue. Basically, you want to make sure that you don't have multiple links that all point to the same page.
Believe it or not, there is a different between "www.mydomain.com" and "mydomain.com." Technically speaking "www." is a subdomain of "mydomain.com". If you wanted, you could actually set up completely different websites on those two different domains!
"Canonicalization" is the process you go through to make sure that only one canonical (or, primary) domain leads to the webpage. Matt Cutts of Google has a good explanation of this topic. There are a number of things you can do to solve this problem, but probably the best solution is to use a 301 redirect of one version to the other.
Should My Domain Name Use www or Not?
This leads to the question - which way is best? Different people in the industry have different opinions, so it can be difficult to know what to do.
I used to prefer domains without the www, but I've changed my mind in recent times. Although I know that you don't need to type in the www, it seems to me that the majority of users are still accustomed to typing the www in first. In my experience, the majority of backlinks coming into websites that haven't set up URL canonicalization use "www." in the link. I think if you decide to go with the non-www version, then you will still have a significant portion of people linking to you with the www version anyways.
Even though you can and should certainly set up URL canonicalization, it's still best practice to have links coming in consistently to the same version of the URL. If all of your links are formatted the same, and you've set up proper URL canonicalization, then there will be overwhelming evidence in favor of your preferred version of the URL. I think you are more likely to get that result if you use the www than if you don't.
Nevertheless, this is such a marginal issue that I wouldn't make a big change if you have already set up your canonical URL without the www. The most important thing is to be completely consistent on your own website.
Just my two cents - what do you think? Do you prefer domain names with or without the "www?"