Andy, at the moment it is very difficult to IPO. Once a corp gets to the required market value, the P/E ration has risen too high. I have seen this in all of my corps in the last week. Either they can't get to a value of 300 or, if they do, the P/E ratio is too high.
It may be that this is a temporary problem as the game readjusts to recent changes - most corps have become unprofitable in the last week and enterprises are losing money. But we have seen this before and I hope that, as before, the balance will re-establish itself.
However, it is worth monitoring in case it doesn't - at which point the players would welcome a GM intervention.
Hugs and respect
Jo