Automatically copy formula to next cell when i enter something in perticular cell

Hi,
I need to copy formula to next cell when i enter something in perticular cell.
i.e


Colomn A --- Colomn B --- Colomn C
1 01-09-07 --- John --- =vlookup(b2,$s$1:$t$10,2,false)
2 01-09-07 --- Smith --- =vlookup(b3,$s$1:$t$10,2,false)
3
4
5


Now if i enter date in cell A3 then cell C3 should be automatically filled/copy formula as celll C2. and so on......
then if i enter data to A4 then cell C4 should be automatically filled/copy formula from cell C3.

how can i do this. any idea?


S.