Just came across a nasty gotcha today, where by we were trying to use the new C# 3.5 syntax
var x = from item in items select new Item { ItemName = item.Name };
and getting a compile error. Turns out this was due to the need to have the following using statement (that doesn't come up in a smart tag prompt when you're writing linq),
using System.Linq;
To be fair the compiler error was suggesting exactly this.