Google has started rolling out a minor software update for Nexus 4, Galaxy Nexus and Nexus 10 devices. There are no details about the update available right now as per a post by T-Mobile, it contains some security improvements. Some even speculate that these security improvements might be related to a recent Bitcoin wallet case.
There is still no sign of update for Nexus 7 or the new Nexus 7 but it can always appear a little later.
Carrying the new Android 4.3 build JWR66Y, the update is around 2MB in size, nothing big. It would automatically appear on your Nexus devices but you can always check updates manually by going to Settings << About Phone << Update.
Update: Also live for Nexus 7 HSPA+ (2012).