I just had the pleasure of watching the Channel 9 video of Eric Meijer (Father of Haskel, champion of XML Literals, and leader of Volta) "interviewing" Bart De Smet (creator of LINQ to Sharepoint, LINQ to AD, and now LINQ to Simpsons). If you have a spare hour and want to dig into the guts of how IQueryable works, I definately recommend checking out the LINQ to Anything video. It's one of the most thorough yet approachable discussions I've seen on the topic. I'll have to watch it 3-4 more times to make sure to soak it all in.
Some of the things they discussed got me thinking about some much broader implementation possiblities. Particularly when they discuss about adding support for statement lambdas, directly passing the expression tree directly to the implementation engine (SQL Server) rather than doing a temporary translation of the query (into TSQL for example), and building a XML based mapping layer rather than brute forcing the expressions via the visitor pattern.
Bart (not Simpson) is definiately a bright guy. I didn't realize he was so young. Time for the mid-life crisis to kick in.