<p>Why are unsigned integers not CLS compliant?</p>

<p>I am starting to think the type specification is just for performance and not for correctness.</p>

Not all languages have the concept of unsigned ints. For example VB 6 had no concept of unsigned ints which I suspect drove the decision of the designers of VB7/7.1 not to implement as well (it's implemented now in VB8).
To quote:
http://msdn.microsoft.com/en-us/library/12a7a7h3.aspx
The CLS was designed to be large enough to include the language
constructs that are commonly needed by developers, yet small enough
that most languages are able to support it. In addition, any language
construct that makes it impossible to rapidly verify the type safety
of code was excluded from the CLS so that all CLS-compliant languages
can produce verifiable code if they choose to do so.
Update: I did wonder about this some years back, and whilst I can't see why a UInt wouldn't be type safety verifiable, I guess the CLS guys had to have a cut off point somewhere as to what would be the baseline minimum number of value types supported. Also when you think about the longer term where more and more languages are being ported to the CLR why force them to implement unsigned ints to gain CLS compliance if there is absolutely no concept, ever?