This question comes up because the mortgage companies don't want to spend the money to have a legal determination that your trust is a revocable trust.
There is no legal reason that you cannot refinance with the home held in trust. That being said, sometimes the hassle factor just isn't worth it.
Many of these companies will agree to retransfer the property back into trust as soon as the refinance is over or you can make those arrangements to have it done yourself. But do ask, because many of the government backed programs do have specific forms that allow you to refinance in the name of a trust.