<p>I need to do some large integer math.  Are there any classes or structs out there that represent a 128-bit integer and implement all of the usual operators?</p>
<p>BTW, I realize that <code>decimal</code> can be used to represent a 96-bit int.</p>

No, there's nothing in .NET <= 3.5. I'm hoping/expecting that
BigInteger
will make its return in .NET 4.0. (It was
cut from .NET 3.5
.)