How is hash rate of mining hardware calculated The mining hardware does a double sha-256 hash of the header while mining.  Is each sha-256 operation counted when determining the hash rate or is the whole sha256(sha256(header)) counted as one hash?

Just to throw out some example numbers, if the hardware can do 300 Mhash/s is the hash rate 300 Mhash/s or is it 150 Mhash/s because two sha-256 operations are required for each attempt?
The sha256(sha256(header)) is commonly called the hash in the Bitcoin world. So the hash rate is the rate of sha256(sha256(header)) operations, not the rate of single sha256 operations. Otherwise it'd probably called a sharate or something similar :)