In reply to Ed Saint: I am trying to find out exactly the same information; certainly it is easy to get to Cortina by bus; there is a regular service from the railhaed at Calalzo; additionally there are some fast services from Venice and Mestre. I know there's an out-and-back service from Cortina to Misurina via Tre Croci. i am not sure if there is a summer service extending from Misurina to Rif Auronzo near Tre Cime di Lavaredo. I have heard somewhere that such a service starts early July. Can't confirm that.
Further west, there's a circular bus several times a day around the Sella group - Sella Ronde bus? (calls at Selva, Colfosco, Corvara, Canazei, sella pass and runs in both directions; Canazei can be reached on a slow but frequent bus from Trento and Selva on a frequent bus from Chiusa. What i can't work out is whether there's a summer service from Cortina, west via Falzarego. to link with the aforementioned Sella Ronde bus.
Can't remember which website is which but try searching on "dolomiti bus", SAD bus, SellaRonde bus, Trentino bus, and a site which has all public transport in the Tirol, both Italy and Austri. Also the trenitalia site. Let me know if you answer any of my missing info.