I have a 2009 Toyota Prius (company car- too Republican to own one myself

) that has auxiliary jack in the center console.
I play my music downloads and actually use RadioTime app to play through vehicle's speakers. When you have the Droid charging through 12v system at same time there is some distortion but that was solved by using a set of
Griffin Noise Canceling Cables.
Through RadioTime I can listen to quite a few terrestrial stations that broadcast over net as well as hundreds of net only stations (like all the Yahoo! stations). I can listed to my local radio stations even when out of town. You do need to obviously have a good data signal.
In my wife's 2004 Honda Odyssey there was no auxiliary jack. I installed an auxiliary unit that hooks to an open 14-pin jack on OEM head unit. The extra port was designed for use by CD changer but was open since none was installed.
This unit is manufactured by PIE and they make units for many different vehicle models.
Amazon.com: Precision Interface Electronics HON98-AUX Connect a portable music player, satellite radio tuner, or DVD player to your factory Honda receiver!: Electronics
The PIE unit has 2 RCA plugs. I ran a RCA to 3.5 mm cable into storage compartment beneath stereo and plugs the Droid into it when I have to drive the van. A suggestion to avoid the interference I have in Prius is not not install the ground wire on the Aux. unit.
Google Navigation, Pandora, My Music, RadioTime, pays through the stereo even if I have a bluetooth headset on.
I have found that when running several apps. (Navigation, Pandora, Radio, music, etc) the Droid battery discharges faster than it could be recharged.