Stephane Eyskens has created a very detailed and extensive article regarding SPMetal and very importantly also the reason behind some of the build-inn limitations.
http://stephaneey.developpez.com/tutoriel/sharepoint/sp2010linqen/
A must read for any who is using or considering using SPMetal for SharePoint 2010
Michael Høtoft
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Sharepoint Server
sharepoint 2010