Before I bought excellent Treo650 I looked many times for application which will synchronize correctly my GSM phone with Palm's addressbook. Because I didn't find any which would meet my requirements I programmed my own application which is now available to all.
After runing GSMsync you need to setup it's operation in menu->setup: select port, baudrate, length for matching phone numbers, addressbook custom item for storing GSMsync information and other options.
Then you can run synchronization: "Sync with Addr". Button "Load from Phone just loads all SIM book, button "Save to Phone" saves all changed items to SIM book.
GSM sync has one add-on function: synchronizing GSM clock with Palm (means from Palm to GSM). I use netClock application for synchonizing Palm's clock with time servers. This time can be then synchronized to GSM phone.
You can try this application and download it here.