Yes, but it sounds like you can do this without a formula. When you create the group on the date field, select "For each month". This will create group values such as 2/2010, 3/2010, 4/2010, etc.
If you need the formula for non-grouping purposes, it would look something like this:
totext(year({Orders.Order Date}),"#",0) & "/" & totext(month({Orders.Order Date}),"#",0)
Keep in mind that if you need to sort on the formula then you need the year first. The grouping function puts the month first, but still sorts correctly.