Fancy title to get things going.
This is one of those things that'll be second nature once I start using it in anger, but until I do that, I'll consistently forget it.
C# 2.0 introduces nullable types, and the null coalescing operator (??) is a binary operator that works pretty much the same way as T-SQL's ISNULL() function.
a ?? b
One small caveat here is that b could of course be a nullable type as well, in which case the result of a ?? b is still a nullable type. If it's not, the result isn't either. See example below (ripped from C# 2.0 spec).
int? x = GetNullableInt();
int? y = GetNullableInt();
int? z = x ?? y;
int i = z ?? -1;