This does not necessarily mean that the steam is radioactive, as the top floor (or what's left of it) of Reactor 3 after the March 14, 2011 explosion remain highly radioactive.
There has been only one measurement of radiation (page 30 of the link), probably from the boom of the crane, above the top floor of Reactor 3, and that is 500 millisieverts/hour (at 8 meters above the floor, if I remember right, but not 100% sure).
According to TEPCO's email notice for the press on July 23, 2013,
TEPCO measured the radiation levels at 25 locations in the area around where the steam has been observed (the entire shield plug).
Maximum was 2,170 millisieverts/hour, minimum was 137 millisieverts/hour.
Radiation level at the location where the steam has been observed was 562 millisieverts/hour.
Radiation levels are simply too high for human workers to investigate on the floor, which is not negotiable at all by robots either because of debris.
(UPDATE) TEPCO just released (7/24/2013) the document containing the measured radiation levels (English labels are by me):
The same page as above, but with visualization, from Tomohiro Endo, nuclear researcher at Nagoya University:
If the steam was coming from inside the Containment Vessel, I would assume the location would see much higher radiation level.