I have a theory that 4.1, regardless of whether it's Jelly Bean or still ICS, will have features that OEM's will be very keen to see implemented, and therefore the adoption rate will jump very quickly.
Either bloggers have forgotten, or weren't around for, the period between 1.6 and 2.2. I seem to recall that adoption rate from 1.6 to 2.1 was quite slow, but 2.1 to 2.2 was very fast and most devices were running it quite quickly.