g1smd - 4:20 pm on Mar 9, 2011 (gmt 0)

Install the "Live HTTP Headers" extension for Firefox and see what you get.

Which bit fails, the redirect from old URL to new URL, or is it that the request for new URL fails to be rewritten to correctly pull the content?

Based on your original URL scheme, shouldn't you be breaking at an underscore, rather than a slash, in some parts of the pattern capture for the redirect?