Do you still remember that good old bucket system in SharePoint Portal Server 2003 where the first 20 portal area's ended up with nice urls, but then the buckets started kicking in and provided you with urls like http:/server/C0/myarea/default.aspx? Those days are over.
The reason behind the bucket system in Portal Server was understandable; Microsoft expected a huge number of areas, with deep hierarchies that could be moved around at will without breaking links, and probably performance was an issue. But the bucket solution is a really machine oriented solution, users want to be able to understand their urls. If not we could use GUID's instead of urls;-) But I think they calculated wrong, I never saw really deep hierarchies appearing, and in most cases not that many areas, because you don't want to do to many things in areas due to all the limitations imposed compared to the standard WSS sites: no list and document library security, no possibility to backup a single area, no possibility to create a template from an existing area, or create an area based on a template... should I continue?
In SharePoint 2007 Microsoft got rid of the bucket system. You can move sites around, and the urls of the sites change!! But what happens to urls pointing to those sites? There is a good side to the story, and a bad side. The good side is that all urls in url fields in lists and metadata are rewritten. This is actually quite cool! Move your site and all lists like links lists are updated!! But the bad thing is that all urls NOT managed by SharePoint will NOT be updated (SharePoint has no control over them), and this will break a lot of links.
I wonder about links within documents within document libraries in SharePoint... would be a nice excercise to update those as well;-)
Another thing I heard, but not verified yet, is that if you put a url in a url field in a list, and this url points to something within SharePoint it is actually stored as a relative url. You can also store a releative url in a url field now, but if you edit the field again it shows the full url, but still stores the url in a relative format.