I think you're correct that the ranking points gained from the victory in San Marino were included in the rankings for February. Otherwise, how else would you have overtaken NI* last month?
However, whilst you are correct in pointing out that these FIFA rankings are entirely separate from the process whereby WC 2010 seedings are decided, they still give a very rough indication, since it's very unlikely that e.g. a country will do very badly in friendlies (count for the rankings, but not WC seedings), but do very well in competitive matches (count for WC seedings as well)
* - If only for the (shortest) month ;)