I might try this:
1
Light the grill and turn the burners to high. Close the grill cover and allow the grill to heat the rusty grill grates for 15 to 20 minutes.
2
Turn the grill off and open the grill.
3
Scrub the grates with the wire grill brush while the grates are hot. Work the brush over the entire surface of the grates until you remove as much of the rust as possible.
4Remove the grates from the grill and rinse the grates with a garden hose to remove rusty dust.
5
Sprinkle a generous amount of baking soda over the warm grill grates and place the grates into an oven preheated to 350 degrees F. Leave the grates in the oven for 10 to 15 minutes, then remove them. The baking soda should cause a chemical reaction on the rust.
6
Place the grates back on the grill and scrub them again with the wire grill brush.
7
Remove the grates from the grill and rinse them again with a garden hose to finish cleaning them.
Try "Magic Eraser"... these things work on just about everything..
http://www.amazon.com/Mr-Clean-Eras...8&qid=1340150262&sr=8-1&keywords=magic+eraser