OK after some digging around in all these threads with weird headings, I was able to find the service combination for F500BT unit.
I found it in this thread. http://avic411.com/forum/viewtopic.php?f=34&t=27581&start=60#p206891.
MAP, MAP, MENU, ENTER (Press down on the joystick), MENU then ENTER.
Here is the other helpful item from that thread. You can only use the RESET button in this service menu.
So to move from option to option, you press RESET quickly one by one. To select, you hold down RESET for 1/2 second or so.
When it ask for you to reset, hold both RESET and POWER for a few seconds until it reset.
I was able to flash the new WinCE and copy over the My Flash Disk from OP. Everything seems to be working so far.
Good luck.
Update!!
Now that I had a chance to fully tested, there is no guidance voice on the F500BT like the two previous F500BT users posted on this thread. I noticed something was weird when the sound volume is really low.
I guess the hacked EZRIDER in this rar file is not fully compatible with F500BT. Guess back to the drawing board.
SUCCESS!! WOWEEEE!! Everything is a success! I found the issue and fix the audio issue.
Please following this instruction for the F500BT model. I was very successful.
You need only a single SD card, at least 2GB in size. Copy the entire contents of the RAR file except for the WINCEIMG.BIN and WINCEIMG.CKS to the root of SD card. For the two WINCE*.* files, I had to use the one I download from this link. http://www.rapidshare.ru/1206567
The one that came with the FX3.1 would not allow me to boot after I flash WinCE>
You will need to go to the folder APL2\iGo\ and delete the current sys.txt and rename sys_749UC.txt to sys.txt.
You can delete the other sys_*****.txt files and any maps that you may not use. You may need to do this because the F500BT may not the capacity for all the new maps.
1. Insert SD card (minimum 2GB) with testmode into AVIC and turn it on.
2. Backup My Flash Disk by copying
3. When the pioneer logo comes up, press and hold the power and reset buttons at the same time until the blue service screen comes up then release power and reset buttons. (you should see Press reset again to reset but don't press reset yet)
4. Press in this order MAP, MAP, MENU, ENTER (down on joystick), MENU then ENTER. (each button press will show an asterisk at the bottom center of the screen). If the correct combination is pressed, then a menu with options pops up.
5. Select Wince Upgrade with Format (SD)
6. At the bottom center will first display the status of reading in the file. It then will clear the message and will show the status of writing the file. Then it will do a quick test. Wait until it says restart in the bottom center. Reset the unit and leave the SD card in (remember to reset you have to press and hold both power and reset buttons.)
7. You should be greeted with testmode (It will look very similar to normal windows). Open Explorer and copy the contents of the "My Flash Drive" Folder from the SD card to the "My Flash Drive" that is located on the unit. Overwrite everything. (It will the copying will take 140min but will only take about 20)
8. On the desktop should be an icon to RESTART the unit. Double click on that to restart, make sure you take out the SD card so normal boot process happens.
8. Select your language and voice options.
9. Enjoy FX3.1 
A special bonus, I was able to download the latest 2010 maps from here http://gpsunderground.com/forum/showthread.php?2688-iGo-8.0-and-8.3-Navteq-And-Teleatlas-Map-Download-Index.-All-here./page25 into the F500BT without any issue. You will need to delete the older maps and just copy the MAPS, POI and BUILDINGS to the respected folder under /APL2/iGO/CONTENT
Really GOOD Luck now and enjoy to those able to succeed.