915 MHz FCC Certification
Absolutely it is required, I assume you are talking about being a radio in the 915Mhz band?  In which case you will be an intentional radiator and will need to go through compliance test at an actual test location, and register the device to get an FCC ID, which you must then apply to your product.

The specific section you are interested in is FCC section 15.249, here is some more information about it from the FCC.

Here are some good app notes, one from TI the other from Semtech.

You'll see that there are also power limits for simple transmission, and higher power limits if you're using spread spectrum.

You will also of course have to pass FCC Class A or B depending on your equipment type.



Edited to answer questions

I'm no FCC or legal expert and you should probably consult one or more of those and a test house.  However if you look here you will see the FCC guidelines to "Determine if device requires authorization under Verification; Declaration of Conformity (DoC); or Certification"  Where they say for "Unlicensed transmitters (low power intentional radiators) – see Section 15.201"

Paragraph 2 of that section says 

"Except as otherwise exempted in
paragraph (c) of this section and in
§15.23 of this part, all intentional radiators
operating under the provisions of
this part shall be certificated by the
Commission pursuant to the procedures
in subpart J of part 2 of this
chapter prior to marketing"

Now subpart J of part 2 states:

"§ 2.907 Certification.
(a) Certification is an equipment authorization
issued by the Commission,
based on representations and test data
submitted by the applicant."

That's your section saying you must apply and provide data for certification, you can read subpart J to learn more about how that testing should be done.  I suppose you could collect the data yourself provided you have access to the tools (and shack or are in a valley somewhere with no interference), and that you can follow an acceptable test procedure.  A little more digging will show how you have to apply along with the fees.

Or you could go to a test house and pay them, which is what most people do.