The link to my first attempt at panning today:
http://www.thephotoforum.com/forum/showthread.php?t=116581
You want manual. Slight varitions in exposure shouldn't be really noticable over a distance of 5-20 feet.
I was shooting at 1/100 and f/9. I'd try to shoot at 1/160 so I wouldn't get so much motion blur. My best suggestion for the actual technique is to us an AI servo mode that keeps focusing and doesn't focus an lock. If the Nikon 70-200 AF's as fast as the Canon's 70-200, then it will really help. Start tracking your target from way before the position you're going to shoot at. Make sure you support your lens from vertical movement as well as possible (monopod can really help here), then fire in high burst mode when you're target reaches where you want to start firing at. Make sure you don't get jumpy at this point. Pick a place on your target and track it and don't jump when then camera starts firing. I usually pick the person's helmet and keep it on that as when they're tucked and leaned off the bike for a turn, you can gaurantee that they're not going to look off in another direction until they're through the turn.
If you can make it to a track where you can practice, then this is the best. With the exception of breaks between the groups, I was getting at least one bike coming by every 15-30 seconds.
EDIT: I don't know if your Nikon is capable of this, but if you can assign a button to AF rather than having a half shutter press AF and splitting it so one button AF's and the half shutter press just meters, then that may help you as well. I tried it today and it really helped.