Rob, I totally agree with the first half of what you said, which is why I noted that I am in fact using the same two players (Music and PowerAmp) that are being used on the problem phone. Yes, the players make a difference and in this case, they are the same players so the players by themselves are not the issue.
As to other applications making a difference, yes, I said that too. The entire bundle of "what's running on my phone?" is going to impact the operation of the phone. Android is kinda notorious, in all versions on all phones, for just going out to lunch and freezing up while it is busy running background tasks. Doesn't matter whose phone you have, if you do enough stuff with it, load enough software, sooner or later you do something simple like try to dial a phone number, and the phone just pretends it is a rock, sometimes for 30 seconds or longer.
This is all very much the same as the problems with every computer OS out there, including all the Mac, Windows, and other *Nix versions. Take a device with a slow processor, dump a lot of tasks on it, and yes some random mixes of it will overcome the device. I know, the Atrix has a "fast" dual-core cpu but it is still chump change compared to what you can dump on it in the way of tasks.
Bottom line is that phones with identical hardware, identical OS, indentical music players, are behaving differently. And that points to the application loads on them as being the problem, as much or more so than just saying the hardware or the code itself is defective.
Remember, Motorola and Google each performed *some* testing on the devices. But all the apps from the Android Market? Are basically untested. Someone writes it, throws it out to the mob, and if no one screams in pain, it goes up on the Market. There's a lot of bad code out everywhere--especially on the Android market, where there really is no quality control until users scream.
I'm just saying, look at the pieces. The applications are the most likely to be written badly. And damn near no one really knows what apps are active on their phone, at any given time. Background refresh? Synch? Sorting? Something turned on a radio? Every time someone says "Gee, my battery is getting chewed up" "Wow, how'd I use four gig of data roaming?" that's someone who doesn't know what their phone is doing in the background.
Sometimes, that's me too.<G> They don't make it easy.