In my thoughts on the threat of mobile VOIP a couple of months ago, there were a couple of capabilities that I thought were blocking widespread adoption and hence reducing the threat to operators. Now both of those blockers are showing signs of moving away.
First, the iPhone 4 launched, bringing multitasking to millions of consumers and hence enabling VOIP applications to run in the background, enabling permenant presence. Second - and vitally - the Library of Congress has made it legal for iPhone (and therefore all) smartphone applications to crack the devices core capabilities (link). Now it is simple for VOIP application providers to enable users to utilise cellular data connections and increase the persistance of their presence as they move around.
Neither of these are mass market capabilities yet, but even so, if I were a mobile Telco they'd represent flashing warning beacons!