Sounds like a very silly question, but I tried, and it isn't too easy to find a simple answer to this simple question on google.
The longer story:
Before I started my search, I just took it for granted that SharePoint provides a web service to access a calendar using iCal.
But after having searched I fear I was totally wrong. I just don't want to believe it. Not even in SharePoint 2010? Twelve years after the standard was published?!
I found a blog about implementing an iCal export, but to be honest, implementing it clean, complete and bugfree, and ideally even two-way is just too much of an effort to start before asking here.
I also found a commercial product called MashPoint, but it's overkill.
The tiny remaining spark of hope makes me put this as a SO question and hope that after the coffee, the whole world looks different all of a sudden.
SharePoint does support iCalanders. I am currently using on an events list so people can save to their outlook.
You can download the .ics based on the url like so:
http://yoursite/_vti_bin/owssvr.dll?CS=109&Cmd=Display&List=%7BListID%7D&CacheControl=1&ID=1&Using=event.ics
Replacing yoursite with your host name, ListID with the ID of your list with the events and the ID=1 with whatever list item number you want.
In SharePoint designer you can implement into the page as a hyperlink like the following example:
http://yoursite/Marketing/_vti_bin/owssvr.dll?CS=109&Cmd=Display&List=%7BA742AAC4%2D2654%2D4E64%2D9280%2D7813D2A343A2%7D&CacheControl=1&ID={$thisNode/@ID}&Using=event.ics
I'm not sure about a direct web service to retrieve the iCal if you want to use pragmatically but i'm sure you can just use this URL and a stream reader to save or manipulate.