(As of end of January 2010)
Here's a table of the number of unique users (by ID - so non-public users are all lumped together) editing in some number of calendar months out of a longer period of calendar months. Along the header row are the lengths, in months, of the period being selected from. Down the first column are the number of months being selected. For example, the number of unique user IDs appearing in at least 3 months over the last 6 full months is in the 6th column, 3rd row - 8,169.
There's no accounting of the editing period, so if a contributor makes only two edits spanning two calendar months (for example at 23:59 on the last day of the month and 00:00 on the first day of the following month) then they count towards the 2nd row. Whereas a contributor may edit heavily for four weeks within a single month and only count towards the 1st row.