I don't know that the .NET "shared source" opening is a trap for Mono, but I do know this: the fact that it is being considered one by some writers demonstrates the depths of mistrust Microsoft has managed to set up around itself. Consider:
Let's say a year from now, Microsoft does a SCO. They claim that Mono contains code that was stolen from the .NET Framework reference source code. They point at their code, they point at the license, and sure enough, there's similar code. After all, both projects are implementing .NET; there will almost certainly be lines of code that looks alike.
Better still, from Microsoft's point of view, all they need to do is find one Mono programmer who has signed the license to look at the .NET Framework reference source code. With that "proof," they'll claim they've found their smoking gun. SCO failed in its attempts because it never did have any evidence that there was Unix code in Linux.
Microsoft, however, is baiting its trap for Mono programmers with .NET cheese. They'll claim, come that day, about how open it was in letting people look, but not touch, their code. With the combination of "proof" that some Mono code has been stolen from Microsoft and its attempt to muddy the waters about what open source really means, it can look forward to having a much better chance of killing off an open-source project than SCO ever had with Linux
I think any such attack on Mono is unlikely, and that it would be a PR disaster if they did make that kind of attack. But never mind - the idea is being rationally discussed. That's a loss for Microsoft right there.