I installed the official Samsung ROM yesterday, and the only thing that wasn't working correctly with ActiveSync. After a few minutes tinkering around, it appears I was able to find the problem.
Go to Setting, Connections, USB Connection Mode... and verify that ActiveSync is setup.
Then go to USB to PC and disable the advanced network functionality.
A quick restart of the Omnia may be required. Once it's back... connect to USB and wait for ActiveSync. Sometimes it still will not automatically detect it, so I have to manually try to connect it. Which it fails through ActiveSync... but I wait 5 seconds.... and the USB detection on the phone kicks in, verifies the USB... then ActiveSync connects. Odd...
GPS is definitely working... I get betwen 4 - 7 GPS satellites, and works within 20 meters. The GPS also works great with the Marathon App. You are able to run/walk laps, and it will tell you how long you have walked/ran, and how many feet... or miles.