Try this conditional format formula,
taken from a post by Tom Ogilvy, re:
http://tinyurl.com/dsu87
" If the hidden rows are cause by applying a filter then this formula will
work:
=MOD(SUBTOTAL(3,$A$1:A1),2)=0
Select the rows to format (this assumes the formatting starts with row 1)
Enter the formula as if it were for the the currently active cell (in this
case A1). The range must be applied to a column that will contain values in
each visible cell in the column. "
The formula above will also take care of the normal unfiltered situation
If it's without filtering, then we could just use:
=MOD(ROW(),2)=0
as the cond format formula