Outside of 3rd party integration apps that can run on iPhone (like Crestron's
Control App for iPhone ) that tend to be tied to expensive back-end systems that can do your dishes and compose symphonies, the free Apple Remote app lets you play/pause, select by playlist/song/genre/etc., randomize, etc. It's basically a remote control app for AppleTV/iTunes, as it's name implies. In fact, I can sit at my iMac and control iTunes using my iPhone, without touching the mouse or the keyboard--why I'd want to do that, I have no idea, though.
I use
AppleTVs in my system--they are the red-headed stepchild in the Apple family. I like them because they are relatively cheap, they do cool photo slideshows (I run photos from each of my AppleTVs into dedicated plasmas for "active art") and they create a relatively nice whole house audio solution, as they synch media, as well as stream from a local Mac. I only do synched music, though--streaming tends to hiccup in my house over my Wi-Fi network. Before the Remote app came along, to control my music, I would have to use the correct AppleTV physical remote, using the correct plasma screen for navigation--Remote changed all that, in a great way (no longer have to be standing in front of a plasma to change a song!).
My A-BUS setup is very basic and low frills--my builder said take it or leave it, so I took it, with some after-the-fact upgrades. Basically, I have
this kit , with additional keypads (and thus additional zones, since keypad = zone amplifier in the world of A-BUS). I'm not 100% thrilled with it, but it works for background music pretty well. I have zones for bedrooms, kitchen, den, TV room, deck, etc.
Again, I have 2 AppleTVs, each set to synch specific media from my iMac (grab photos from iPhoto, grab music from iTunes). iTunes itself *cannot* handle multiple *distinct* streams (you can stream iTunes to multiple Apple
Airport Express modules, albeit each ones gets the same stream), so the synched AppleTVs offer an easy solution to this problem--each one pumps out it's own music, so no worries. There are numerous 3rd party products that can do
multiple distinct streams , but they tend to cost more than doing multiple AppleTVs and offer less--not more--flexibility, so what's the point?
As noted, in theory, the Remote app supports volume control of sorts, but it's extremely wonky in my experience (it's a slider that doesn't seem to respond well to tactile input), it's only for streamed iTunes or streamed/synched AppleTV audio and, worst of all, it seems to want to revert to 100% volume each time the app starts (in other words, you'd probably blow your amp/speakers relying on it to handle volume).
So, if you want to go the iPhone/iTunes Remote route, you're going to need another way to change sources and handle volume--that's where zone keypads come in. If you only have one source (e.g., an AppleTV or streamed music from your iTunes library, perhaps using something like an Apple Airport Express since your Mac isn't likely going to be sitting next to the back-end hub/amp that drives your whole house audio system), you don't need to change sources, so you could get away with simple
volume attenuators in each zone--think volume controls, with a central amp handling the power (i.e., you plug your music source into an amp, then run speaker wire to each volume control, then speaker wire from the volume control to the speakers for the zone).
I have set up 3 different whole house audio systems for friends/family in the past 6 months, each leveraging Apple gear. Two are bare bones (e.g., iTunes streaming wirelessly to Airport Express, Airport Express connected to a receiver, receiver connected to impedance-matching speaker selector for safety, speaker selector connected to zone volume attenuators, connected to zone speakers, all controlled by Apple Remote) and one is like my system (A-BUS, multiple AppleTVs as sources).
I'm sure people can/will chime in that you can do the same--or better--with a Windows or Linux solution. I just find Apple "easy" in this regard, and as noted, I find AppleTVs to be cheap music (and photo and video) servers. The addition of the Remote app seals the deal for me--that level of wireless control used to literally cost thousands of bucks, and now it's free.