I think it's safe to assume that it is, in fact, 7200rpm. The specs list the non-Fusion HDs as 7200rpm. I seriously doubt they'd pull a switcheroo and use different, slower drives for the Fusion options, given that the upgrade cost to Fusion is the same $250 as it is for all the 21" iMac and Mac mini configurations for which it's available.
Another reason: the reason Mac mini and 21" iMac use 5400rpm drives is because they're using 2.5" laptop drives in those. The 27" is using 3.5" drives (the existence of a 3 TB option proves this) and with 3.5" drives there's no real cost saving in using a 5400rpm.