We are here at TechEd EMEA 2008 in Barcelona and have just attended a really cool session by Bart De Smet (blog here) titled LINQ to Anything. As you can imagine, the talk was about writing Linq providers to any data sources. Bart has active projects on LINQ to ActiveDirectory and LINQ to Sharepoint which really gives a good understanding of what it involves writing a custom provider.
The really interesting question is, are anyone working on a LINQ to Sitecore project? or how about an template entity mapper (like to one in LINQ to Sharepoint)? Give me a shout if you are interested – maybe we can get a open source project going on the topic.