The Archdiocese of Milwaukee statistics published annually in the (print-only) Directory supplement to the Milwaukee Catholic Herald show a Catholic population of 643,775 in the just-published 2010 edition, compared to 681,781 in the 2009 edition of a year ago. That's a decrease of 38,006 or 5.5%.
I have not seen an Archdiocesan explanation of these statistics. This might not reflect an actual year-to-year decrease, but rather that recent parish censuses finally showed previously unreported declines. But that is only a question of what the rate of decline has been, not what will be done to reverse it. If anything.