I might get trapped with I'm going to say, but it's what I feel.
It seems to me that Mr. Nichols is a bit misguided in his reasoning. First, Microsoft opening .NET is not different for Mono (giving the conditions) as if Microsoft kept the code under a veil, safe from prying eyes. Second, it's not a trap. Microsoft 'sued' the entire Linux community because they suspected that the kernel contained copyrighted code from Microsoft, and it turned all was for naught. A similar stunt with Mono will go nowhere, IMHO. Third, Mono is primarily developed by Novell, a company that most people has a cooperation agreement with Microsoft. Fourth, .NET extending to Linux and Mac is all the better for Microsoft, as for one day they want to open more fields of business (OK, the Mac is already open). Fifth, there's reverse engineering

. And sixth, god damn, Miguel de Icaza already said the Mono community should not take a look at .NET Framework source code, I know it's tempting but... In Mr. Nichols defense, it should be noted that both entries were published the same day, so it's natural he wasn't up to date in Mono movements regarding the matter.
BTW, another use for this code opening is hinted at one of employees at Microsoft
in his blog. This one came via a complicated interconnection of links, so your head may spin if I detail the entire path (mine is spinning now

)
I'll go to a safe place now

EDIT: Altered the first sentence in the second paragraph as it was unnecessarily harsh and product of a fast reaction to the article.