Bing takes forever to follow redirects and index the new URL for a page.
Searchengines never forget a URL. They will recrawl every URL they have ever seen forever, even after it rediects, or returns 404 or 410.
Pierre Far mentioned at SMX London that Google recrawls 404 and 410 pages and that those returning 410 are recrawled less often. Google continues to look at these pages because a significant number of them eventually come back into use returning real content served with 200 OK status.