I disagree with Peter G, I think the primary driver has to be financial - its the fundamental behind being in business. The advantage of moving to DTV was the ability to broadcasting channels and created additional revenue. The move from MPEG2 to MPEG4 would require the same incentive.
I think there are two potential motivators. Political and financial. IIR, the last time around it was generally the government directing, paying and handling the digital change over. As we near the end of that transition I can't see the government itching to start the process off all over again. So I agree the only likelihood of a fast switch to MPEG4 is if the networks see a decent return in it (which there's not an obvious lot of it.) As I mentioned before, if a network could some how show horn an HD MPEG4 primary channel broadcast into its spectrum (which retaining the SD MPEG2 one) then that might see a ratings benefit.