So far these are my findings:
I should mention first that I am of course rooted and have frozen all VCast apps in Titanium - I don't know if that makes a difference, but thought I'd mention it.
On Windows XP without the Samsung drivers (they have never been installed on this particular pc) when I plugged it in with debugging on, it popped up the new hardware found - search for a device driver on the computer and the usb mounting option showed up in the notification bar of the phone, but of course, without the driver Windows thinks it needs, it wouldn't be able to see my files even if I mounted it. I made the mistake of closing the search for driver window before unplugging my phone and my screen went black, softkeys still lit - frozen....had to do a battery pull.
When I turned off debugging and plugged it back into the XP pc, I then got the Internet Explorer pop up, which I immediatly closed, followed by the prompt to install VZW's crappy software, which I declined and after that auto play started, which I cancelled and then the option to view files on my phone was given to me. Pulled down my notification bar, mounted my sd card and was able to see all files on my phone.
My experience with Windows 7 without the Samsung drivers was a direct mimic of what I've been hearing here already - no reaction from my phone when plugged in with debugging off (no option to mount from the notification bar) and Win 7 quickly told me it could not find the drivers for my device. Plugged it back in with debugging on and got the box offering the bootstrap.exe or whatever that was, but no option in my notification bar to mount the sd card. That's as far as I went with it because it seemed like there were plenty of instances of this already documented.
I guess it's a really good thing that I almost never use usb mounting or I'd be as ticked as many of you are. For now, it still mounts perfectly to my Ubuntu, so I'm a happy girl.