Now then, this is a complicated situation this 7.6.1 update problem!
I have iPhone 3G running latest iOS, Airport Express (not sure what generation but couple of years old and a 'N', Airport Extreme again not sure what generation, about 18 months old? Plus Macbook unibody 2009 running Snow Leopard. I have absolutely no problems with 7.6.1 - streams from iOS to Express no problem, all clients show excellent connection (54-114Mb/s) on iOS Airport Utility app.
In contrast I have just spent two days trying to sort out my friends setup. He has iPod Touch (about two years old) running latest iOS and iPhone 4 running same, Airport Express (about 6 months old) Airport Extreme (same age), same Macbook but on Leopard.
Since 7.6.1 he can't stream from iPhone at all - just refuses to start Airplay. Touch works better, sometimes for as long as 10 minutes, then just stops. Internet is effected as well, over wi-fi speed is .5Mb/s on Ethernet around 5Mb/s. Macbook for the first time ever shows less than full bars on the fan icon, investigating with iOS Airport Utility shows connections have as little as 1Mb/s in some case and vary wildly as you watch.
Experience as shown that 5Ghz network is fine if named and chosen as a separate network and connected to by the Macbook, full bars, 300Mb/s excellent connection, Internet not interfered with - all good - obviously this doesn't help iOS devices. Selecting 2.4Ghz network immediately reduces signal strength. All of these issues point to a much reduced 2.4Ghz signal strength emanating from the Extreme after 7.6.1 update - and I can probably assume from that the problems connecting for the iOS devices on this network are down to local interference that was not a problem before this reduction in signal strength. We always had a problem when using BT Home Hub as a wireless router with constantly being knocked off by nearby routers, this was completely solved by the introduction of the Extreme base stationl.
So, I conclude Apple have seriously cocked up some setting in the firmware update that affects transmitter strength and further this cannot be reversed by rolling back the software - well not in my friends case anyway. He is now on 7.5.2 on both his Express and his Extreme and although network reliability is much improved (iOS devices couldn't even connect on 7.6.1) the airplay streaming issue is the same and internet at 2.4Ghz is effected.
End of story... we shall await further developments Apple!