It is possible to sync Google Contacts accross BIS services
filed in Mobile Productivity on Oct.20, 2009

Over the weekend many users have reported seeing the “Contacts” options start appearing on their Gmail accounts while logged into their carrier’s BIS sites. Telus, AT&T, Rogers, T-Mobile and Verizon have all been accounted for at this time and I’m sure many others are to follow. This was one of the highly noted upgrades to BIS 2.8 that was stated as coming approximately 2 weeks after launch.
