referenceing 'last row' in a cell formula
 

I know how to do this in a vba script, but not as part of a cell formula ...

Given a sheet where the first few rows contain some fixed 'header' and
'summary' information, then below that are 'detail' rows that are added to on
a daily basis. I have a cell in the 'header' area that I'd like to build a
formula that references "column a" in the last detail row.

Thanks in advance