Some HTC EVO 4G users have been reporting that in order to get the latest OTA update (version 4.67.651.3), they unrooted their phones, switched to S-ON, and then took the OTA update expecting to re-root their phones on the other side. Unfortunately what happened is they discovered that Revolutionary.io does not support the new HTC HBOOT . . . and that leaves them stuck with a stock unrooted phone.
Whether HTC changed anything security-wise with the HBOOT requiring new exploit code or the company just changed version numbers, Revolutionary will reportedly refuse to touch it.
While we don't doubt that a root method for the new update will be found eventually, right now we recommend that you do not 1) unroot your phone if you're rooted or 2) update via OTA if you're planning to root. You can still get the latest OTA from your favorite custom ROM developer.
When the official Gingerbread update was first released for the HTC EVO 4G, it took two months before a root exploit was engineered, and that was when a lot more developers were focused intensely on the device. As the original EVO is now coming up on its second birthday, there are significantly fewer developers still working on it, and HTCDEV has not listed the EVO 4G as something they're going to unlock.