Actually, older FM tuners seem to handle things better than the newer ones for whatever reason.

Honestly, I'd recommend doing everything you can to secure an aux in of some kind (most cars can do this through the CD-charger port), as the quality of both FM transmitters and cassette adaptors is well below par.

I use my N800 in my Jeep all the time, just not quite in the way everyone else seems to approach it. I've got an asus Wireless router (Wl-500Gd) that's running OpenWRT and Music Player Daemon on it. The router has USB ports so all my music is stored right on cheap thumb drives that I've got plugged into it; the router is powered by a small DC-DC adapter that's wired into the 12V supply for the cigarette lighter. From there it's just a matter of using your favourite MPD interface to setup playlists and things; any time you want more music/etc either I unplug one of the USB drives and throw some more stuff in it from my computer when I go in at the end of the day, or Wifi-it over from the tablet using the default Samba file sharing support in OS 2008.
Being a Jeep and all there really aren't a lot of input options and I didn't want to replace the head unit as in the summer when I've got the roof off the thing would last for about 5 minutes before being stolen, so I picked up a HiFast USB audio card/FM transmitter combo, and the thing is awesome. No interference problems (I can get clear signal at about 100 feet, non-line-of-sight, so gives you an idea of how the thing compares to something like those whimpy iTrips/etc) and several frequencies to select in case you suddenly park next to a transmitter for a radio station or something...
It's a bit more work, but if you're serious about in-car music it makes a lot more sense then relying on your tablet.
1) The thing is always present and powered only off the vehicle, so battery life over time is not a concern.
2) Much more expandable thanks to USB. I know the tablets recently got USB host mode to work, but see the next point...
3) It's not mobile. Sure, it's easy enough to unplug the three cables from the back when I want to take it out, but it's not something that I have to do everytime I get in and out of the vehicle. Also, since I don't have to interact with it directly I have it mounted out of the way which keeps the interior space nice and clean (especially important for me again because of the no roof/theft problem and because quite frankly there just isn't a lot of space in a Jeep!)
4) Rugged. The tablets are great, but if I'm offroading or even just out on rough roads I don't want to be worried about my lovely tablet getting bounced around, falling out of it's cradle, or worse getting full of dust in the summer! Since I use the tablet only to control the router, I only need to have it out when I want to change something; once that's done it's safely stowed away again.
5) Passenger friendly. Because the tablet's not anchored or wired to anything, I can hand it out to anyone else in the vehicle who can take their time to scroll through and choose music/set up playlists etc with the tablet in their hands, as it was designed to be. They can make all the music choices and changes in realtime without having to plug it back in etc.
6) Tactile control. I also rigged up a few physical buttons to the router that control basic playback with some simple scripts. This is by far the biggest advantage, as it lets me have basic playback control without having to look down at a screen.

Anyways, I know it's probably more of a personalized setup than you're looking for but I thought I'd share it since it's my version of what you're trying to do.