I mentioned this in my last blog entry, but thought I should make it more discoverable. We've released the May Community Tech Preview for Linq. Everything I talked about here at Devteach is available for the download.
I just spoke to an attendee named David who asked me whether we support adding nodes, etc., to the query tree or creating your own version of DLinq for custom data sources. The answer is yes! The documentation is in the CTP - and there is a great blog entry by Matt Warren here. Hope this helps!