The island of Chebeague in Casco Bay (near Portland) has Wi-Fi: Two nearby islands will also gain ubiquitous coverage through Ubiquitair. The fees are a little odd, as you pay $40 per month for unlimited access within their coverage area, but only $30 per month if you stick an antenna on your house if you're outside the 10 to 15 acres and some businesses they broadcast around.
The folks behind the service aren't looking to make a fortune, but believe that summer usage from the population quintupling will be the equivalent of Christmas sales for the retail world.
Over in Midcoast, an area defined by what it's between--Bath on the southeast to Searsport on the north--Midcoast Internet Solutions has been running long-range wireless since about 1997. They offer coverage to a few marine islands and many towns that are effectively inland islands with little connectivity.