I quadruple-Googled it just to make sure (and I'm still not 100%!!) but as I understand it, traditionally you use an 's' for the verb, e.g. I practise playing football, and a 'c' for the noun, e.g. a doctor's practice, but Americans use a 'c' for either.
It's one of these grey areas according to usage. The same rule supposedly applies to license/licence, although it seems more acceptable to use either spelling interchangeably. On your driving licence it's definitely spelt with a 'c' so I'll always spell it that way for the noun, but I wouldn't correct anyone using an 's'.
Edited: 21/12/2012 at 10:42