A while back Inbar Gazit blogged about a new (at the time) Orcas type System.Numeric.BigInteger. BitInteger is intended to represent an integer of “arbitrary” precision–from 1 digit to 2,147,483,647 digits. “Arbitrary” because realisitically approaching 2,147,483,647 will not have enough memory to store the integer.
For whatever reason, this type didn’t make it into the RTM version of .NET 3.5–it’s actually internal, not public. I don’t know why, Inbar didn’t follow up.