I actually think the demand will (as always) outstrip SE's expectations.
Even after Beta P4, the forums, their account site, and everything else related, has remained slow as hell. This leaves me to believe that they will either have an above-normal Open-Beta-New-Player-Retention-Rate; OR... their servers are literally on fire and not yet fully recovered from the madness that was Phase 4.

I hope and pray it all goes well, but I have also learned to know better when it comes to expectations and reality...