Even if the Headers and/or Footers are turned off in Document Margins of a Pages '08 (v. 3.02), the values that were in these fields when the check boxes were checked become the minimum top/bottom margins for the page.

If your margins appear too large for what you have specified in the top/bottom margin fields, turn on the header/footer margins, type in a number less than or equal to your desired margin, and then turn the header/footer back off. Your margins should now match what you have put into the top/bottom margin fields.