updated 04:40 pm EST, Wed February 6, 2013
No differences spotted
(Update with info on "File:///" fix) Apple is distributing a tenth beta of OS X 10.8.3 to developers, listed as build 12D65. As with the previous beta, there appear to be no significant code changes. Apple is also asking developers to test the same components as before, including AirPlay, AirPort, Game Center, graphics drivers, and Safari.
The development of v10.8.3 has been one of the most protracted for an Apple maintenance update. By a tenth beta the company would normally have released the update to the public, which suggests that the company may be having a particularly difficult time with some bugs.
Update: Some developers are now saying that the build appears to fix the File:/// bug recently discovered in Mountain Lion.