I got the How Clean is Your House book for xmas, Kim and Aggie say this on Iron Maintenance:
Keep the sole plate clean and free from scratches by wiping and then polishing with a soft damp cloth (when the iron is switched off and cool). If the sole plate picks up burnt-on sticky residue, don't scrape or scour it off. Instead, heat it up to the warm setting and rub with a tightly stretched, dampened, loose weave cotton cloth or towel. If residue remains, rub with white toothpaste or bicarbonate of soda and a coarse damp cloth.
Hope that helps xx