Sorry if you feel it's a little bit off topic, but I thought I'd write it here since it's closely related to SMW.
I created Google Calendar widget for my Widgets extension and Yaron reminded me of SMW's iCal export (which didn't work with Google Calendar when I originally tried it). I updated my SMW installation to the latest version and, lo and behold, it works! Long URLs are still driving all readers (rss and ical) crazy, but I'm just using cron jobs to save them as static files (maybe it's time for those "saved queries" e discussed on dev list).
So, creating iCals for your SMW instance works with Google Calendar and you can use Google Calendar Widget to embed them back to your SMW wiki or to any other wiki:
You can see it in use on TechPresentations.org for Current and Upcoming conferences: