Interesting question. Most likely Sony will be releasing an update to 4.0 but it may take some time.
From reading, the official launch of 4.0 / Ice Cream is tomorrow, 11/10/2011 but that wont really mean anything since then manufacturers will tweak it for their devices. Samsung apparently are planning to roll it out for their devices in a few weeks so I can assume that after Sony have finished, it shouldn't be too long either.
Even with the P due for launch, since their are only small little adjustments needed (assumption) to compensate for two screens, I can not really see it being a 3/4 month wait for a new o/s but from what I have read up, it will be more an intergration betwen mobile and tablet apps more than anything else so the trasition should be pretty subtle.
