Ugh, I know what you mean!
My DD started this a few weeks ago. She has always been really good w/car rides, but has become very whiny suddenly (and she's got a great car seat - wish I had one that comfy!).
I think it's because she is so incredibly mobile now and considers it downright inhumane of us to force her to be stationary for a few minutes, lol. I will say the last couple days has gotten a LOT better, she has started walking the last couple of weeks and as she is mastering this milestone it's getting better. Today before taking her home from my parent's house (a 35-40 minute drive) I had her walk up and down the sidewalk for a good 10-15 minutes. It really helped, seemed to get the urge under control for the ride home lol. I know it's not practical for every errand, but perhaps try something really physical before hopping in the car?
I echo what isaac's mom says as well.. I now take our 4 magic CDs EVERYWHERE with us HUGE help. Also, (and I was not too keen on just giving snacks for entertainment purposes) but I give her an organic teething biscuit which she will just gnaw on for a good 15-20minutes,. It's a bit messy but it works!
Good luck, your cute little guy will hopefully be through w/this phase (and on to who knows what!) pretty soon....
Tari