i hear this all the time but i think its BS. i have had systems online with no patches for a week and nothing happens.
Then something upstream from you protected you. Some ISP's will filter malicoius traffic and ports. If your ISP did this you would be much less likely to get infected (your infectors would be the subset of infected users using the same isp's vs everyone).
Otherwise, no, it's not BS. There have been plenty of studies about this (my company does one yearly). And it's simple to reproduce, put up a honeypot and see how many different attacks you get within a day.
I used to throw up an open ftp server to see how long from the port was open until it was used as a warez dump (I also had a service running which randomly corrupted any files put into the ftp directory, making sure the warez dump wasn't usefull). Shortest time was about 1/2 an hour, longest was 2 days.
Bill