Looks like some sites are open according to the status page, and advanced reservations are showing up in rec.gov:
FLA, MCD, SNY
COS,ELH,GAB (curious ELH but not ELF)
BOW, KIN, QUA
ATL, REF, REH
I'm curious as to how the walkup availability will be shown, as I'm assuming the old server page won't be available. The majority of sites that don't have reservation availability show 'W' for walkin, although clearly some of those aren't available for walkin (STO, BOU, etc). There are a couple exceptions, such as BRO, HOL, LQU showing '0' until their standard opening date. So clearly some suspicious values so I'm not putting too much stock in them. The one thing I did notice is that there are only 3 parameters: total, remaining, show walkup. The total matches the total available for advanced reservation. The available is the available advanced reservation, up to the total value. And show walkup determines whether the 'W' is displayed. I haven't found one where there is available and show walkin = true, but have found where there is 0 available and show walkin = false which results in the '0' showing up.
It is a bit too early to tell, but it appears that walkin will have the 'W" if there is indeed walkin available, but it won't be reservable on rec.gov, but also won't display how many sites are available for walkup.