It's not only the OS that needs to be ready. There are many of us outside of RIM who are positive it is, but there are still things that can go wrong. Working in IT myself, I am fully cognizant of the s*** that can happen no matter how many times you have tested or simulated deployment. We have had this occur many times ourselves. Successfully testing a deployment numerous times in a lab or dev environment is no guarantee it will work in production no matter how closely you have simulated your production environment. This is why I always shake my head in disbelief when someone encounters a bug in newly released software and feels the need to add the comment "typical (insert company), didn't they test this thing before release...". Of course they did, but unless one of the testers duplicated the conditions exactly, then the devs won't know anything about it. You can't test every possible combination of factors and conditions.
In addition to the OS they are updating PB App World and Bridge for the phones. This adds additional variables and increased load on both the Update servers and App World servers. App World will become busy not only with the Bridge updates, but the newly updated Playbooks as well due to people suddenly discovering hundreds to thousands of apps now available to them. There are also quite a few more Playbooks out there since the last OS update in December, so I'm sure their IT department has been busy adding new servers and performing load tests. I don't for one moment expect any kind of major disaster, but I do expect the odd hiccup. It just goes with the territory, S*** happens, and sometimes you have no control of it.
Here's hoping for the best.