My wife's MacBook is running 10.5.8. According to Apple, in 10.5.3 (and in all others after that, I assume), there is a checkbox in AddressBook's prefs to allow syncing to Google Calendar and contacts. However, on my wife's Mac, there is .Mac and Yahoo, but not Google.
I'm running OS 10.6.2 and I have the Google checkbox.
I found a hack (published just before 10.5.3 came out) that is supposed to fix the problem (change a value in com.apple.ipod.plist), but that didn't work.