I think I found an update to 10.1 already

I don't know if this is for real, but I was using limewire the other day and came across something called "Mac_OS_X_10.1_5G48_Update." I downloaded it and it wouldn't open, but I was wondering if anyone else has heard of this. If so, what have you heard? What does this update? Why won't it work with the software update control panel?

Mac OS 10.1 is 5G64. 5G48 is an older build from the betas that led to 10.1. If there are really newer builds of OS X around, they would either have a higher number at the first position (6Xxx) or a later character (5Jxx).
If you open the About this Mac window and click the text reading "Version 10.1" you'll see that it's aka "Build 5G64."

-another IA with a tin cup & pencils
If it's under 10Mbs, it's some fake thing that's been going around limewire. Just pitch it - I have no idea what it really is.