Apple today released version 10.8.4 of OS X Mountain Lion. The update, which is available now through Software Update on the Mac App Store, brings bug fixes and improvements. Notably, 10.8.4 includes Safari 6.0.5, a fix for connecting to enterprise WiFi networks, improved FaceTime calls to non-US phone numbers, and Exchange improvements in Calendar.
On a dedicated webpage, Apple mentions that a popular issue of iMessage and the Messages app has been fixed with today's update: "A fix for an issue that may cause iMessages to display out of order in Messages".
Apple has posted direct links for the 10.8.4 update -- as usual, both in standard and Combo versions. You can get them through the links below.