I have the option to do the over the air 3.0 update from AT&T for a HTC Inspire 4G.
Will I lose root?
Generically speaking, if you've only rooted the device and haven't actually installed another ROM, you should be able to apply the OTA update, but you will lose root because of it. You can always root again.
This is mentioned on several forum threads:
As one can see by the posts on each one of the above threads, there's a device agnostic consensus. Users report that OTA updates are possible on rooted devices, but root is always lost.
The reason behind this is that updates essentially replace or add system files, and the permissions on these files that the rooting process has changed to give you root access, are changed to the ones that come with the update, that don't allow root access.
You can also try using an app such as Voodoo OTA Rootkeeper to protect your root. On supported phones, it can preserve root through an OTA update. It must be installed and set up before you apply the OTA update.