It's linear and we measured it with a laser tachometer a little while ago. There's never been an official word from Festool though.
They're a constant step speed, so on the OF 1400, approximately...
10,000 RPM 1
12,400 RPM 2
14,800 RPM 3
17,200 RPM 4
19,600 RPM 5
22,000 RPM 6
See
http://festoolownersgroup.com/ask-festool/router-speeds/ for some more info.
