It is possible to go with your iPod as a source unit. Kicker has an in-line volume control, the ZXM-RLC that has just started shipping. It is an active volume control, so you go from zero volts at "all the way down" to a max of 9 volts, assuming a 4.5 volt input from a traditional head unit. Your iPod will put out less than 4.5 volts, but know that the ZXM-RLC provides a 2x voltage bump compared with wiring the iPod straight to the amp.
So, you install the amp. Then install the ZXM-RLC so that it is connected to the RCA inputs of the amp. There are actually two zones of control, so you might want the second zone to drive something else, or to provide independent control over two amps, like for front and rear, or in boat and tower... Then hook your iPod to the ZXM-RLC input and voila' you have a nice iPod based sound system without a head unit, and you still have good volume control over the whole rig.
To take it the rest of the way there, find a 12-volt accessory charging device that will keep your iPod charged while plugged in. There are many out there, and some of them go so far as to actually have a line level RCA stereo output from the plug on the bottom, rather than the headphone jack. This will sound better for most applications.
David at Earmark shoud be receiving some ZXM-RLC's any day now. Hook up with him to see about buying one.