Greg,

Do you mean that the Notes subform includes a date field?  So each Note
entry has a DateEntered or DateCreated or DateUpdated or whatever?  If
so, put an unbound textbox (hidden, probably) in the Header or Footer
section of the subform, with its Control Source set to the equivalent of...
 =Max([YourNotesDateField])
Let's say you name this textbox LatestNote.  Then, on the main form, put
another unbound textbox, with its Control Source like this...
 =[Notes]![LatestNote]
or, as some would have it...
 =[Notes].[Form]![LatestNote]

If you don't have a date field in the Notes, then I have missed the
point - please let us have more details.