On the MS website there are 2 things you can download.
1) March 2006 update - default.zip (http://www.xbox.com/en-GB/games/backwardscompatibility.htm). Running the default.xex inside appears to just drop a $systemupdate folder (and update) on the hard disk and a $titleupdate folder (with an update for the emulator). This is fine, I deleted the system update since I obviously don't want/need this, and the title update did apply (confirmed by differences on the emulator error screen) - great up to march 2006.
2) December 2007 - $SystemUpdate_12-2007.zip (http://www.microsoft.com/downloads/details.aspx?FamilyID=603aa2f7-bbca-4fa6-b679-794bca845577&DisplayLang=en). This is just a system update as far as I can tell, no title updates. So is the emulator update part of the system software now? I'd be surprised, and if it was wouldn't it be in the version xbreboot ships with?
I know I don't have the latest version because it tells me Outrun 2 isn't supported, but that's listed as a new addition on the 2007 compatibility list.