nokia named various versions of interface/os using S**. I think they started out with S10 or S20 for their B/W LCD nmt and gsm phones, then moved on to S40, then licensed symbian and created S60 (and a brief S70, or S50, that they used as the basis for the maemo1-4 ui).

strange btw, as i thought nokia's stated plan was to drop S40 and use S60 on all non-maemo/meego products.