Undocumented Linq! .NET 3.5 SP1 Gold Forgot to Mention these Critical Changes that might Break your Code
BREAKING CHANGE: Cast<T> Behavior
Also, a new implementation recognizes queries that apply Where and/or Select to arrays or List<T> and fold pipelines of multiple enumerable objects into single specialized enumerables.
According to Dinesh Kulkari at Microsoft this enhancement: "….produces substantial improvement in base overhead of common LINQ to Objects queries (at times 30+%)."