Which Tuple class do you mean? The only Tuple around I know about will be part of next .NET Framework 4.0 release (see
Tuple class pre-release documentation).
In general, if some feature was new in 2.0 and it is not part of 1.1 (e.g. generics) and you need it on 1.1, then you will have to work around it. For example, create your own Tuple or use object with casting instead of having a generic class. There is no simple easy automated way how to "downgrade" your sources or binaries from 2.0 to 1.1.
-Karel