In 2006 the ferry cost USD 48 one way and USD 96 return and ran Mondays and Thursdays, departing Mersin at 9 AM and returning from Latakia at 2 PM. I will update this post when I hear the prices and schedule for this year.
Thank you Christel for the tip!
UPDATE: The 2007 ferry schedule is the same as 2006's: departing Mersin at 9 AM on Mondays and Thursdays and returning from Latakia in the afternoon on the same day. The cost is $65 one-way and $130 return, more than 30% more expensive than last year. Merlat sell tickets: telephone +90-324-237 1668.
NOTE: some advice from Adam in comments:
...these ferries don't seem terribly reliable ... the one on Monday 13 August 2007 was cancelled altogether. Check and double check, if you're thinking of using it!