Writing the ModRewrite rules is the easy part, you would use something like:
RewriteRule ^/.*/[^/]+\_([0-9]+).html+$ index.php?a=1002&b=$1
the rules are ignoring the "Category" part of the string anyway. The more difficult task would be getting the site to leave the "Category" off of every URL string.
For SEO purposes this is a good idea as spiders will find the page closer to the root and therefore expect it to be of higher value.
-Travis
http://www.bestcodingpractices.com