Phil,
I'd be really careful with using foil to line the drip pan. If you're grilling something fatty, the grease could potentially hang up in the creases of the foil. Having a build up of grease in the drip pan is not a good idea.
You could have a real messy grease fire. At best, you're meal is ruined. At worst, you might have a serious fire.
If you do line the drip tray, make sure you take your time to line it without bumps and creases.