Talk to your local county extension agent, or to animal control. Either one should know who in your area keeps bees. Call them and see if they can come look at the nest, and decide if it is honey-bees or another variety. Even if the bees aren't honey bees, I bet they would help move them. Unless the nest is wasps or hornets, which should probably be killed, as they can be VERY aggressive starting about now (both only live for the summer - with queens overwintering to start a new nest in the spring), I would think they would be willing to move them. Honey bees and wasps/hornets are - to my knowledge - the only ones who make large nests.

In any case, I am sure no reputable tree removal co. will be willing to cut down the tree without you either moving the nest or killing the bees - too much liability, esp. if someone is severely allergic and unaware.