An update and review...
After searching quite a bit, on and off, at the Apple store (web) I discovered the Kensington LiquidAUX Deluxe. It has a cradle on a flexible stalk attached to the "power outlet" plug, which is perhaps its only real weakness as there's some small inevitable "play" in the plug's fit. Not much, however, and the LiquidAUX stalk is much stiffer than the TuneFlex -- the phone has never moved on its own under acceleration or braking, so I don't need to constantly re-adjust it.
As the name implies, it has a cable (nylon sheathed) to plug into a car stereo AUX input jack. If you need an FM broadcast adapter for your car, this isn't it.
The cradle has adjustable arms to grasp narrower iPods, and the iPod socket actually slides back and forth in a track to accommodate models where the socket isn't in the middle. (It moves fairly easily, and can sometimes get knocked a little to one side when you pull out your iPhone; a very minor annoyance, but I wish that the "action" on the slider was a bit stiffer so this wouldn't happen as much.)
The LiquidAUX Deluxe also has a small remote control designed to be velcro-ed around the steering wheel rim. It has play/pause, forward, back, and "shuffle" buttons. I've noticed that the remote doesn't do anything at all unless iPod is running on the iPhone. That is, after a restart or sync, when everything is idle, the play button won't do anything. I have to manually bring up the iPod program and press "play". After this, however, iPod always has some component running in the background, apparently, and the remote will (usually) continue the playlist even after I've pulled the iPhone out of the cradle, carried it around "doing various things", and finally return to the car to stick it back in the cradle. Mostly, it's nice while driving and watching the GPS to be able to skip or replay a song without looking down to either switch out of the Map application or do the "iPod double click" to bring up the control dialog; and for that it seems to be quite reliable.
Kensington also has a "LiquidAUX" -- apparently the only difference for the Deluxe version is the cradle. The non-Deluxe has the charger cord, AUX cord, and remote. The whole thing is a bit pricy at $99.99, and I wish the Deluxe was at the price point of the non-Deluxe ($79.99), which is already more (really, sigh, quite a bit more) than I really wanted to pay. On the other hand, the warning dialogs and the waving stalk on the TuneFlex were slowly driving me crazy and this was the first really viable contender... I just "had to".
LiquidAUX Deluxe charges the 3G without any "incompatibility" dialog box. The remote is sometimes useful, and at least it's out of the way. The flexible stalk raises the iPhone to a level where it's easy to see the GPS (or an incoming call) without losing sight of the road -- although I wish it were a little longer. (I think it's stiff enough that another inch or two probably wouldn't subject it to the TuneFlex's "waving in the wind" troubles.)
I've been using it for several weeks now, and I have to say that despite the minor quirks regarding the remote control (more likely to be due to the iPhone software behavior than the remote) and the "slightly lower than ideal" position in the car, I'm quite happy with this device. Ultimately, I think it was worthwhile, even though there's room for improvement.