Read the link for more details. A few highlights:
The FCC has not yet released a Public Notice on this. (keep an eye on the FCC website.) Usually, this kind of proceeding takes effect 30 days after publication in the Federal Register; said publication will take between a few days and a week. So one might expect AM stations to begin operating in digital mode at night in about 5 weeks, around the beginning of May. I would not rule out the possibility of stations that are already operating IBOC during the day receiving Special Temporary Authority to engage in nighttime digital operation before the new rules go into effect. Indeed, I would not rule out blanket Special Temporary Authority to cover all IBOC stations.
Many of the 50kw clear-channel stations are already equipped for IBOC and will likely begin operating at night as soon as the rules go into effect. The likely result will be massive interference in the 640-1220 and 1500-1580KHz bands. I don't see the new rules spurring many stations to add IBOC though, if they aren't already running it. IBOC is not particularly popular among the smaller stations in 540-630, 1230-1490, or above 1580KHz and I don't think it will become so particularly quickly.
I guess time will tell.
FCC News Release
Deborah Taylor Tate
Radio World article
another Radio World article