Typically the later models of head units support total Ipod control where the song titles scroll the display and FF, RW and shuffle are controlled by the head unit buttons. Look at Alpine, Blaupunkt and Kenwood, et. al. for specific features. Surf
http://www.crutchfield.com for the specifics. Alpine seems to have the best selection that are Ipod ready.
Other models of MP3 players are typically not supported for full control, but many head units can take an AUX input where the headphone jack of the MP3 player can be plumbed into the unit. This is OK, but not optimal because you have to fiddle with both components.