From their online specs:
1TB (7200-rpm) hard drive
Configurable to 3TB hard drive, 1TB or 3TB Fusion Drive, or 768GB of flash storage.
http://www.apple.com/imac/specs/
Im talking about the Fusion Drive, not the simple harddrive.
There's no specs saying a 7200rpm will be included with Fusion Drive.
They might choose a 5400rpm when you buy a Fusion Drive duo to heat or something