You have an answer, but since I spent some time on it, here's mine! 9^)

require 'csv'
csvData = CSV.readlines("d:\\ruby\\dev\\filldown-csv\\filldown.txt")
puts 'Before:'
csvData.each {|l| p l}

1.upto(csvData.size - 1){ |i|
    0.upto(csvData[i].size - 1){|j|
	csvData[i][j] ||= csvData[i-1][j]
    }
}
puts 'After:'
csvData.each {|l| p l}


For some reason my CSV wouldn't read the data when it has quotes around
the values...

cheers
Chris