Greekofile.
There are normally two catamarans that leave Heraklion for Santorini covering each day but not both running every day and also the standard ferries, but I'm not certain how often they are. The ferry companies are notorious for not releasing their schedules or opening up the on-line booking until the last minute, but sailing times are generally the same from year to year.
We got the SeaJet MegaJet (or something like) which was scheduled to leave Heraklion the day we went at about 8.15am (I think it runs most days but sometimes just after 7.00 am) & there is also the Hellenic Seaways Flying Cat which I think left the port at 9.45am in 2009 (but doesn't run on some Wednesdays & we went on a Wednesday!).
Now I believe the Flying Cat generally starts in mid April & the SeaJet in mid May (as I reckon the later caters more for the coach/hotel day-trippers). Both companies have websites but as above they won't tell you much at this stage.
We booked our tickets a few days before departure at a travel agent in Ag Nik without any problem & IIRC correctly it was €41 each single from Heraklion to Santorini. I would imagine this will be pretty similar for 2010, but even the ferry companies won't know yet.
Hope that helps.
Pete