Denatured alcohol lightly on a rag would be worth a try. Don't use laquer thinner or acetone as they're too agressive and will attack plastics right away. Dawn might work too but just don't use Power Dissolve Dawn as it's very agressive also (I inadvertantly stripped the paint off my barbeque with PDD).
And if you need a reason to upgrade to another iPod, you could try just putting it in the dishwasher!
