Apple doesn't offically support the V800 phone yet, but a slightly modified iSync works fine. Essentially you need to alter the iSync binaries to treat the phone like it would the already-supported K700. After working out how to do this, I wrote a script that'll do the modifications. To make the changes, download the script to your Desktop then run it by typing the following into Terminal.app

$ cd ~/Desktop
$ sudo sh change.sh

It will ask for your normal login password. After you reload iSync, you should now be able to add the V800 via the Devices -> Add Device menu.

[robg adds: I haven't tested this one, but I did check the script to make sure it doesn't do anything more than it claims to do...]