The clever people at Spanning Sync are working on a service which will allow for full, two-way syncing between Google Calendar and iCal.
This has long been a Holy Grail for Mac users. Now it looks as if it could be in reach, judging from the screencast of a beta version 
and the screenshots at Spanning Sync.
According to TUAW a private beta test is just starting. When the app launches you will find it here .
[Via TUAW ]
[...] the unofficial apple weblog (TUAW) has a great story on a potentially killer app for mac users who also like to use google calendar within gmail for collaborative calendaring and do NOT want to use .Mac services for this purpose.. [...]
If they can achieve this successfully, I will be SO happy. For a guy who loves using his Mac, but has to use Windows at work, this sync solution will be fantastic.
[...] Syncing iCal with Google Calendar: “ [...]
[...] Hawk Wings » Blog Archive » Syncing iCal with Google Calendar [...]
When can we expect a public beta or a release of this seemingly incredible app.
According to the web page, “soon”.
More helpfully, the Spanning Sync blog suggests that good progress is being made with the current private beta
.
Two-way gcal/ical sync tool: http://gcaldaemon.sourceforge.net
Here’s another open source tool that’s OS X specific:
Check out http://jinsync.com
And it works. And it’s a public beta now too :-D