According to Neowin, the most noteworthy change to Build 4417 is that Redmond has fine-tuned and polished the Back-Stage view feature. The latest leak packs a new upload center to store and share documents online, a new activation system, and updated icons. Users are reporting that the uninstall issues of a previously leaked Mondo build have been corrected, and the current version can be uninstalled.
Microsoft Office 2010 first leaked to torrent sites back in May, and Microsoft pushed out a technical preview on July 13. I have no doubt there will be many more unofficial builds to come. If you're going to partake in that scene, be careful of shady sources -- it's not uncommon for these leaks to be laced with malware.