Synthesis AG - SyncML Client for iPhone and iPod TouchUnfortunately gCal is not on the list, Goolge just added CalDAV support, but not SyncML (see Palm, 2005 and Wikipedia CalDAV -- I wonder if the SyncML spec has too many issues).
Synthesis SyncML Client for iPhone/iPod Touch OS brings SyncML standard based data synchronisation to iPhone OS X based devices.
This allows mobile over-the-air (OTA) data synchronisation with any compliant SyncML server (such as Oracle Collaboration Suite, goosync.com, consolidate.at, Open-Xchange, MDaemon, eGroupware, ZYB, mobical, DeskNow, ScheduleWorld, neopim.com and many more)...
So this doesn't help with my real interest -- direct sync from the iPhone calendar app to Google Calendar.
More interestingly, the Synthesis team tells us that the Apple SDK limits what SyncML can do:
... At this time, Synthesis SyncML Client for iPhone is offered as a free preview in the App Store synchronizing the Addressbook. Current restrictions in Apple's SDK for the iPhone OS do not allow accessing calendar, notes or emails...As is usual with iPhone SDK limitations, we don't know if this will be remedied with time or whether Apple prefers to keep the competition at bay.