Just updated the firmware on an AirPort Express and AirPort Extreme from 7.5.2 to 7.6. All working fine after the update.
There appeared to be a hiccup with the Extreme as it said there was a problem with PPPoE, however trying Safari the internet was working, using FTTC, so I quit AirPort Utility and all is OK.
Looking at Apple Discussions some people seem to be having problems, everything alright here, though; I am on SL 10.6.8.
Updated both an Airport Extreme (2nd generation) and an Airport Express from a PPC machine running Leopard, with no problems so far. The MacBook Pro and iPhone seem to connect a little quicker but that could be my imagination. :)
I've had the odd network blip with a 1st generation Time Machine (yes it's still working) which I use as a network extender and backup but it's been fine since.