Here is what ive found that works for me so far, after digging around on the Verizon forums.
Without USB debugging on and usb mode set as mass storage, plugin to USB and the autorun CD rom asking if you want to install tool bootstrap pops up. Let it sit for about 30-40 seconds and it disappears, then the usb icon on the phone show up and the pull down menu will then show usb connected. Choose mount, wait another 10-15 seconds and it mounts.
Hope this helps others out.
Its really ridiculous that it took almost a week after the update to figure out a simple thing like how to move files back and forth to my F##$%G SD card.
Luckily I often check Android Central to find V Fascinate news, otherwise I would have discovered this bug unprepared.
The quoted solution (from post 69?) seems to be working for me on my XP netbook (still need to try on my 32bit Vista laptop) with a minor variation:
Instead of waiting for the installation request to go away, I started it and then canceled in the middle. after a few seconds I got the option to mount as USB mass storage device through the phone, just like it did pre dl09 update.
The only continuing annoyance is that each time I connect explorer pops up (my DEFAULT browser is firefox so go figure) with an invitation to install vcas crapware...
BTW besides this really annoying bug, dl09 is great, my GPS is locking faster than ever and I can open PDFs straight from my email without saving first to the SD card
now come on Froyo - I need support for hebrew fonts without rooting!