Figured it out...
I used this formula to define the default value of the date picker control :
addDays(today(), -((number(substring(today(), 9, 2)) + 1)))
This takes today's date (Oct 25th) , and deducts the number of days passed in the current month (25), plus one extra (i.e. 26)
Thus returning the last day of the previous month (i.e. Sep 30th)
With the date picker formatted to mmmm yyyy, this shows as the previous month (September 2012)
(Although be aware / warned, the full value is technically the last day of the previous month, i.e, Sep 30th...)
APMOB