Kevin's idea is fine for cleaning but it will not replace the white paint
grout reviver pen on eBay (end time 19-Jul-10 20:01:15 BST)
Loads of the stuff on ebay and it's basically a very tough white paint that is applied through a felt tip nib and dries almost instantly. ( I may give it a go on my dads dentures ) You can also use it for tyre lettering if you like that kind of thing.
Greg
PS. My new printer is excellent at copying £20 notes, Is it legal ? :-)