If your year starts in January, then the simplest thing to do is use this calculated field:
MONTH([Your Date])< MONTH(NOW())
which will compare dates to current month and return True or False for each row, depending on its date. Put it on the filter shelf and select only True.
Dimitri - I use the following because it also considers the year:
datetrunc('month', [Date Field]) = datetrunc('month', TODAY())
Never mind, I should reread the original post. =)
Thanks! Dimitri, your answer worked great!