Since ArcGIS 10.0, I have noticed that using Field Calculator on selected rows while in Layout mode causes all rows to be calculated, not just the selected ones. The behavior when in Data View calculates values for selected rows only. This looks like a bug to me.
Thank you all for your testing. This is embarrassing! Now I can;t reproduce this unexpected behavior of layout mode field calculator on my computer at home. My observation of this problem has been on the ArcMap installations at Tufts. Sorry for the false alarm. I'm marking this question as answered. -pbc