Generally, yes. As Claudia said, you'll want to review any prepayment rider and ask the loan officer you are working with. Most conforming loans don't have a prepayment penalty, but it never hurts to be certain.
If you know with a great deal of certainty that you will not be in the home long after the rate becomes adjustable, these loans can be attractive. I'm seeing 5/1's with rates in the middle 2% range with a nice rebate. But, as others have said, if you are thinking there is a chance you'll be in the home longer it's not hard to find something in the 4% and below range and you can save the cost of the refinance.