Just wanted to say I ran through this process after reading the entire thread as well as 90% of the WM5 upgrade Threads and the How to set up the phone as a USB modem thread and everything worked perfectly the very first time through. Thanks SD and mrailing as well as everyone else to contributed to this thread. Just thought I would list some of the things I did that may have contributed to my first time success.

Started with full VZ WM5 upgrade. (speedstep not installed...yet)
dowloaded and installed samsung MCCI drivers (reset computer)
hooked phone up to computer in USB modem mode and let windows automatcally install the modem (reset computer)
dowloaded and installed Samsung PST light
dowloaded and installed samsung i730 PST plugin (reset computer)
downloaded YH25 image file (using the supplied vzwshop link and unzipping the .exe file with winzip
Uninstalled Activesync before "downgrading" phone stack (reset computer)
unplugged power cable to cradle before "downgrading" phone stack
reset phone (practiced putting the phone into emergency mode several times)
followed SD & Cr0n_J0b's instructions
Did not change any com port settings
used the bent paperclip trick to put the phone in emergency pda mode in the cradle
Everything worked from there

Thanks again