Now, in .NET Framework 4, Microsoft decided to add it back (check out the .NET Framework 4 poster). For me, it's a big WOW. Long time ago I need to develop a library which need to store very big integer values and convert from decimal to binary, to hexa and vice versa. And it's a painful experience. Even I used this class but it doesn't work properly for very very very big integer values.
Nice to see BigInteger now :-)