tedster - 4:16 am on Nov 28, 2012 (gmt 0)
Pages that have a noindex meta tag or x-robots tag are still fully crawled - the content just does not show in the SERPs.
In practice, I have never seen evidence that the number noindex URLs directly affects the ranking of indexed URLs. Theoretically I can see that a large percentage of noindex URLs could create a problem. I've never worked with a site where the percentage of noindex pages was high at all, so I can't say for sure.
From the experience of several sites who tried noindexing some URLs while to cope with Panda, once the noindex was removed, good ranking wasn't immediate but it also didn't seem to be as slow as a brand new page might be. For example, in one case decent search traffic appeared after three weeks or so. Obviously, YMMV but I don't foresee any absolute barrier here.