Most succinct LINQ To SQL for taking COUNT(*) of either side of many-to-many?

Answer 1

 from pta in db.PostTagAssoc
 group pta by pta.PostID into t
 select new {PostID = t.Key, TagCount=t.Count()}

Answer 2

This will also show those posts with 0 tags, and is a bit clearer in my opinion.

from p in db.Post select new {PostID = p.ID, TagCount = t.PostTagAssoc.Count}