How many licks does it take to determine which operating system has more updates posted (on average)? There's absolutely NO QUESTION as far as my experiences go: Ubuntu.
Every month I open all of my virtual machines in VMware and apply updates to each to ensure they're in good shape. I have an Ubuntu 8.04 VM running as a Gnome desktop (not a server), a couple of Windows Server 2008 and Windows XP instances, and two Windows Vista instances. This month alone there were 154 updates waiting to be installed on my Ubuntu 8.04 client.
So for all the folks who've emailed me over the last year about my rants on Linux, I'm really sorry. Does that mean Ubuntu is "worse" than Vista (whatever that would mean), no. It just proves my relentless points about software being "soft" and therefore in a state of flux.