I saw in the SAP documentation that only C++ is supported for creating plugins for UDOs. My question is if they mean only "Visual C++ 6.0" (i saw a screenshot and it looked like v6.0). Or does it work on any C++ platform like Visual C++/.NET ?
I suppose you are talking about the optional implementation DLL file, right?
There's nothing that should stop you from using VC++ .NET as the resulting DLL file does not require .NET framework installed - unless you include .NET libraries.
VC++ 6.0 is simply a commonly used development tool in R&D - that's why you saw it in some screenshot...
I hope this answers your question.