Poorly designed content management systems (CMSs) can cause more problems for your website than any other single factor.
Your choice of CMS is therefore one of the most important decisions you can make.
The problem with them is that they tend to prioritise the ease of page generation over everything else, something that can play havoc with search engine friendly-ness.
Most e-commerce scripts and simple CMSs now have "SEO friendly" stamped on the box.
Unfortunately most of these are decidedly not SEO friendly. Some of them might offer things like custom title tags and meta tags but this is as far as most systems go.
The real problems arise when the CMS starts creating thousands of useless extra pages on your site. This can cause huge problems and drag your Google rankings down for your real pages.
SEOmoz has an interesting article today asking whether you really need a CMS for your website.
The The simple fact is that most business owners want a CMS so that they can avoid paying a designer to keep the site updated.
Unfortunately sometimes the SEO cost of using a CMS is more than the cost of the designer's fees.
A simple CMS can be altered and configured to be 100% SEO friendly but sometimes, especially on complex e-commerce sites, this is almost impossible.
In cases like this it always pays to have your site reviewed for SEO before it goes live.