I work for a large local government organisation who are about to embark on using SharePoint to replace our ageing intranet with an all-singing all-dancing collaborative sit
I don't agree with Martin that the high licensing cost is the main issue. The main issue is that Sharepoint isn't designed to be a CMS for public facing websites. Ever looked at the HTML that Sharepoint spits out by default? You can fix all those issues but I've spoken to people who did that and they all say that it doesn't make sense to use Sharepoint for your public facing website. So if accessibility and interoperability are important don't use Sharepoint.
I'll agree that sharepoint is great to build an intranet.
I think umbraco is the best cms for public facing websites. It's designed to have full control over the input, you can integrate any website design (No limitations) and you can use your existing .net controls. That's why I've chosen Umbraco as the default CMS for my customers in 2007.
Cheers,
Richard