i also used the tape adapter with poor results, then switched to the I-trip.
after spending quite a bit if time on ipodlounge.com, i was able to work around the quirks that the transmitter poses. one of the biggest things i found was that if the area that you live in is bombarded with FM signals, you will generally have poor results. while i'm in metro SLC, i'm down on the in the public radio station frequencies, once i'm out of the metro area, any clear station will work.
i would also be interested in a plug in option (as i use my ipod quite a bit), that will work with the stock head unit/six disc changer.
i also read on ipodlounge, that there is a way to boost the FM frequency modulator. in a sense making your Ipod a mini radio station, the article talked about being able to transmit your music to cars driving next to you etc...