Apparently people have, and apple's done nothing about it. I've found articles talking about developers thinking it's a conspiracy by apple to sabatoge web apps because then apple doesn't get the 30% cut from the App Store. :P
I found a page which mentions something that might explain it: daringfireball.net/2011/03/nitro_ios_43
"The real answer is about security. Perhaps the biggest reason for Nitro?s performance improvements over WebKit?s previous JavaScript engine is the use of a JIT ? ?Just-In-Time? compilation? A JIT requires the ability to mark memory pages in RAM as executable, but, iOS, as a security measure, does not allow pages in memory to be marked as executable. This is a significant and serious security policy. Most modern operating systems do allow pages in memory to be marked as executable ? including Mac OS X, Windows, and (I believe) Android1. iOS 4.3 makes an exception to this policy, but the exception is specifically limited to Mobile Safari."
That was iOS 4.3 though, so I'm not sure it applies anymore. It sounds plausible though that the extra security could be hampering nitro. Anyone know?