Is Airport Express compatible with AirPlay? I have an Airport Express that I have plugged into a speaker system. I can change the music in my house with my iPhone through Apple's Remote app. Great. 

Now is my Airport Express compatible with AirPlay? Can I 'push' music or sound to my Airport Express and out the speakers that are plugged into it? Or is this only possible with an Apple TV? And if I get an Apple TV can I do what I'm trying to do? 'Push' music from someone's iPhone that is on the same WIFI network as mine to a speaker system?
With iOS 4.2 you can send music directly from your iPhone/iPad to your Airport Express from any application supporting it.
You can also stream both audio and video from your iDevices from Apples applications.
Starting with iOS 4.3 you can also stream video from any application, not just applications developed by Apple.