I've been playing with 2.3 some more and updated my 2nd AppleTV to 2.3. Here are my conclusions about AirTunes issues:
Keep in mind that I stream my entire music and music video library from my PowerMac in my den (using two 500GB Sata drives, one to back up the other with Carbon Copy Clone) so something may behave differently if you store it locally. IF I go to the AirTunes menu from the main menu and select my upstairs AppleTV unit and then deselect the dowstairs unit (the one I'm controlling) and go and try to play a song, it will fail to start playing most of the attempts, but one time did start and stopped shortly thereafter and then locked up. Once it stops 'attempting' to connect to the upstairs ATV, I can go back to unselect that unit and reselect the downstairs one. At that point, if you start to play a song, it goes to a black screen and appears to lock up, but in actuality, if you wait long enough (a few minutes), it will eventually come up with an error message stating it can't play that song. In fact, no song will play at this point. Music artwork wasn't coming up so I suspected the Mac in the den had crashed but it hadn't. I had to play a locally stored video to get audio back and then music would play again (otherwise, I'd have to reboot the unit).
That describes the failure of AirTunes control from ATV and it's a repeatable BUG as far as I can tell. Now IF I select the play the upstairs room IN ADDITION to (instead of in replacement of) the downstairs unit, both units will successfully play without issue. I forgot to try to then remove the upstairs unit and see if continues playing after both start playing. I'll try that next. But the problem appears to be related to removing the ATV you're using before starting to play a song. It may also be related to streaming and I also can't dismiss the possibility something else was going on since it did start playing that one time but then stopped and locked up shortly thereafter, but plays fine when selecting to play both. Finally, AirTunes always works fine from iTunes on the Mac.
I've tried out the new remote control feature and it's pretty slick (a learning component instead of a learning remote! What a novel reversal). I've set three other remotes to be able to control it, which means whatever other stereo component I happen to be using can control it so I don't have to put that remote down if I switch to AppleTV.
I don't know what people are talking about when they say standby isn't on the menu anymore. It's under general preference settings and works the same as before. Holding the play/pause button down from the main menu also goes to standby as before.
I still wish they would add a visualizer option for music playback (would be great for Pink Floyd and a nice change from endless slide shows). Clearly, the unit is capable of running Safari just fine (i.e. hacks prove it) so now that ATV has iPhone/Touch remote support, you'd think they'd add their own official browser support. Given the HD resolutions, it'd work great on my 93" projector/screen. I shouldn't have to hack the thing to get what should be obvious future features.
Edit:
I just tried my upstairs audio-only system since upgrading the ATV and the latest "Remote" for the iPod Touch. I still see a delay for artwork sometimes when accessing an AppleTV directly, but sooner or later it starts working. But the best addition I noticed since updating the ATV units is now there *IS* a "Speakers" bar across the "Now Playing" screen, making it much easier to control the rest of the house via Airtunes. It's also now present in the "Settings" section for AppleTV units (formerly was only in the iTunes Library control section).
Unfortunately, direct control from the ATV does NOT have a "computer" speaker option. I can only control other AppleTV and Airport Express speakers from an ATV. If you want to access the computer, you have to use the main iTunes library. Even then, it can only control THAT specific computer. My MBP running iTunes and PC running iTunes do not appear in the speaker lists. Only the computer you are accessing directly.
One other nice new feature that appeared on the iPod Touch was the ability to create and edit new playlists, but only for the computer libraries. It does not function on the AppleTV units. It's also still not as straight forward as Signal's playlist, which you can add to as you play and without a separate playlist. On the other hand, you can save the iTunes playlist and even add new ones directly from the iPod Touch (or iPhone).
I'd still like to see the ability to play music from any of my music libraries directly TO the iPod Touch itself (through headphones) without having to have that music stored on the iPod Touch. Similarly, it'd be cool to be able to stream music videos and movies TO the iPod Touch from my iTunes library. And of course, it'd be nice if you could AirTunes stream FROM the iPod's own music library to any AirTunes device around the house. These are the major features still missing from the equation (along with a visualizer for both AppleTV and the iPhone/Touch).