Hi Jeff,
The 301 Redirect is not reported as an Error in the Pro App, but a Notice.
You can see this in the Crawl Diagnostics Summary where you see the big Red, Blue & Yellow buttons.
Notices are listed in the yellow section.
For each of the sections there is a short explanation and the Notices section reads as follows:
Notices are interesting facts about your pages we found while crawling.
So what this means is that items listed in the Notices section are simply highlighted to ensure that you are aware they exist. The 301 redirect from the non www root domain to the www sub-domain is normal practice to ensure that the pages in your site are not seen as duplicates of each other by search engines. So, there is no need to be concerned about the 301 redirect highlighted in your Pro App Report.
The Errors(red) and Warnings(blue) are the issues that can have an impact on the SEO health of your site.
The explanatory notes explain this:
Errors - Pages with crawl errors are often difficult for search engines to crawl, index and rank.
Issues listed as Errors should be dealt with and eliminated as quickly as possible.
Warnings - Pages with crawl warnings are often penalized by search engines.
Issues listed as Warnings may be a problem for your site, but in some cases (for example where you know that you have added 302 Redirects for a specific purpose) you may just say to yourself, "yes, that is the way I want it to be" and choose to ignore them.
When you are looking at each individual page in the Pro App reports, you will find a tiny blue Help link at the top right of each page (directly above the export link) which will take you to an in-depth explanation of that particular Pro App feature.
Also, if you are ever concerned about something in your reports, you can always ask a question here in Q&A or email the Help team direct at help@seomoz.org. Those guys are awesome and always happy to help out, as are the many members & Associates who lend a hand here in Q&A.
Staff also cruise through Q&A from time to time, but with so many questions posted each day, if you have an issue that needs staff input it is a good idea to email the help team and make them aware of it as soon as possible.
I hope that helps.
Sha