Griffin Tech made a USB based audio system that can be used with the Apple Pro Speakers. It actually has that same connector and provides power for the speakers. Unfortunately, it is no longer offered, but maybe you'll see one on eBay. It's called the
PowerWave. I have one.
http://www.msnbc.msn.com/id/4969110/
It connects by USB and is powered through USB and through an external power adapter. It has input and output connections galore (an audio Swiss Army Knife). That connection for the Apple Pro Speaker can be used with an included breakout cable for connecting a pair of regular +living room+ audio system stereo speakers (the unpowered type). I have mine connected to decent large speakers with the other RCA connector output connected to a stand-alone sub-roofer. Pretty nice sound.
I bought it back when I was using a G3 iMac, but it still works fine on my Intel iMac.
Note: Griffin also makes a device called FireWave. It does not provide powered output for speakers like PowerWave.