[zappa24]

Whether you can defer admissions at all is up to the department as was mentioned by the last poster. Some schools won't allow it, saying that you'll need to take your chances and reapply. Others will allow you to defer admissions to a later year. Whether you can defer funding is a completely different story. It's probably likely that your department will give your funding to another student and force you to compete for funding again even if they do allow you to defer. So make sure to ask your department not only if they will allow you to defer admissions, but also if you will still be guaranteed funding when you return.

Edit: I forgot that you said it was a fellowship. If it is a university-wide fellowship, it might even be less of a guarantee that it will be there when you come back. In this case, definitely confirm that not only can you defer admissions, but whether you can defer the fellowship (it would suck if you deferred and then discovered that your funding wasn't there.)