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Since this question was asked the {compiler} R package was introduced. I wrote a post describing what I have found this far regarding the use of JIT and R:
http://www.r-statistics.com/2012/04/speed-up-your-r-code-using-a-just-in-time-jit-compiler/
The post describes some advances since what Dirk wrote in his reply (several of the links in the post are actually to Dirk's newer articles regarding this topic)
I hope this help future R people searching on this topic.