[Update 4/3/2011] Bethesda has confirmed that a new title update has been rolled out in all territories, so the save game bug should be fixed now.
[Original Story] Like ray-ee-ain on your wedding day, Bethesda's poor luck with Fallout: New Vegas bugs is a little bit ironic. The v1.3 patch, which was meant to fix a variety of stability issues, has actually caused another issue on the Xbox 360, causing save data issues for some users.
Complaints on the Bethesda forums (via Joystiq) aren't universal, but those who are experiencing issues say they can't load save games since the patch went live. Bethesda has responded, saying they are "working with Microsoft to resolve this issue as quickly as possible."
For now, your best bet seems to be reject the patch if you haven't already accepted it. Or you could let it roll and take your chances, if you're the type to gamble with your save games. Vegas, baby!