worked for me
With this iTunes 8.1 glitch with calendar I found one simple solution for me.
in iTunes,
-select your phone under device,
-click 'info' tab,
-under 'advanced' check 'calendars' under replace information on this iPhone
-click 'apply'
this will replace the information in your phone with the desktop apps information, it will also install the new defaults for calender colors, etc, that were absent before,
you will loose entries that you made in your phone since the last sync, you might want to jot those down before you do this.
worked easily enough for me, hope it helps