First off, there is an Ai-Net (Alpine standard connecter) to RCA adapter. Alpine just doesn't advertise it since it's cheap unlike the iPod link.
That said, I can't say I'm all that fond of Alpine HUs. The cheap ones have a bunch of junky features, and you have to move up to BIG dollars to get features that are acually useful (auto volume adjust, cell mute, max volume at startup, etc.)
When it was car stereo time for my Vette, I picked the cheapest HU at Crutchfield.com which had good sound quality, aux input available, RDS, auto volume adjust, and max volume on startup. I ended up with the Blaupunkt CD52 Casablanca and couldn't be happier. It's got both amplified outputs and preouts, two aux inputs, capability for a remote if you're that kind of person, and can be wired for illumination, speed controlled volume, cell phone mute, and antenna up/down. Graphically, it's very nice too without being too "bling"y. It's got a graphic EQ and dynamically adjustible EQ settings along with the usual balance/fade/bass/treble. Removable faceplate too.
http://www.blaupunktusa.com/BlaupunktUSA/Products/Archive/Casablanca%20CD52.htmIt's nice too because the bezel is removable, making the actual fitting smaller than DIN. Came in handy when fitting it plus my RioCar (slightly larger than DIN) into the opening in my Corvette which wasn't quite double-DIN.