My VBulletin forum uses the VBSEO sitemap generator. You can find it at vbulletin.org or at the vbseo home page. Currently my forum is generating 15 sitemaps every night. 14 of them have 49,000 urls each. For my wordpress blog I am using the Google XML Sitemaps generator. But its only submitting about 800 urls in one sitemap. I dont have but about 165 post on that blog. So there is a long way to go before I hit 50,000 urls there.
Very good answer, you must have a very large forum, I currently have three sitemaps, one for each subdomain, although none of them is up to 1000 urls, it took me a hard time to realise that I can actually have more than 1 sitemap