Give the Headus UVLayout demo a try. It has an option which takes into consideration a model's subdivision when UVing the cage, hopefully preventing or at least lessening the stretched areas you're getting.
I may be asking the obvious, but did you try rendering it without mentalray? As a Maya/MR user, I would suspect it's a setting in MR. No idea how approximation and the like are implemented in Max, though. @renderhjs, sianki and Valander -- thanks for the thoughts on the 3rd party tools. Sounds like UVLayout and Topogun are…
What's the most popular UV layout plugin or standalone that Max users use? Roadkill? Do folks consider UVLayout and Unfold3D to be worth the $$$? Any opinions on these external tools for Max? Also, on a similar subject, is there any need for Topogun, etc., with the new Freeform tools in Max 2010? Does Topogun do anything…