I just came back from Korea and I have to say the decision to fly to Jeju Island was certainly a good one!
Anyway, I was searching to forums, trying to find if anyone had toured the area with an english speaking taxi-guide. There was no information so I resorted to asking a family friend for help and the tour guide we got in the end was really good and the tour couldn't be any better.
It is wise to travel Jeju (if you're there on the first time) with a taxi-guide, especially if you don't speak korean. We engaged the services of Sunny, a local who speaks fantastic english, and embarked on our 2 day tour. The three in my group were driven around the island and took as long as we needed at each stop, and we expressed our interests in local delicacies and our meal stops were planned strategically at all the best eateries around the island. I believe we covered all the major sites, and all that could have been done in 2 days had been done.
Anyway, I understand that only a few local guides speak English in Jeju, so I took the liberty of getting Sunny's contact details. If I'm not mistaken she charges 250,000 won per day and she drives. You can reach her at 010-2968-6821 or email@example.com