We are often asked what the difference is between www.abc.com and the non www version (http://abc.com)  Most people don't realize there is a difference and that it does matter if incoming links are split and pointing to oth of them 

It's called a canical domain issue.  While Google is working to alleviate such problems in the future, it is important that companies be aware o this issue.  Does it cause problems?  Yes.  Can it be fixed?  Yes, without too much effort, your IT department or an SEO company like Vertical Leap will fix the problem.  Is it critical?  It depends on whether or not ranking is important to you.  Why create more issues by having Google see these incoming links as going to two different web sites when you don't have to?

It is a problem because you want each relevant link to add the maximum value to your web site. You can lose this advantage and have the another potential problem of Google viewing each site separately and penalizing you for having duplicate content.

How do I fix this problem?  Make sure all incoming links are linking using the www.   Also you may want to use a 301 redirect to solve the incoming links problem as well as making sure that you are not penalized for the appearance of having duplicate content.