In a Google Docs Spreadsheet, I would expect this formula:
=ARRAYFORMULA(ROW())
to fill the column like:
[ 1]
[ 2]
[ 3]
[ 4]
The following may be a little simpler:
=arrayformula(ROW(INDIRECT("A"&ROW()&":A")))
or
=index(ROW(INDIRECT("A"&ROW()&":A")))
You need to specify a cell-range argument for ROW() in order to have more than one value.
Try it this way:
=ARRAYFORMULA(ROW(A1:A10))
This will fill a column with row numbers from 1 to 10.