The Mac L4 is based on .NET/Mono. Mono is open source from Novell and there are cross licenses with Microsoft.
1. There is some concern that the cross platform license was only for 5 years with no future guarantees of continued licensing from Microsoft.
2. Furthermore, some have expressed concern about Novell's market slide and current search for a buyer. Will Mono continue to be supported?
Are any of these concerns justified?
Yes.
Are concerns justified for using .NET on Windows? Yes too, just not as much risk.
1. Continued Mono: Not soo much; It is to MS's benefit that they have a cross-platform VM. It increases the "stickiness" of Windows/.NET. (could write volumes here)
2. Support: Definitely a concern, but not much IMHO.
Fact is, things change. If you're looking for a sure thing, old fashioned books are still the closest... even then there's plenty of risk (they easily get lost, spill drinks, pages are fragile, etc.).