Yep...I've used that bus while transferring flights at PHL in August 2011.

As I like to point out: did airside buses even exist before 9/11? They seem like a quick-fix patch put in place to allow transferring passengers to avoid the TSA.

I never did an inter-terminal transfer before 9/11, but I'd think that having to exit and re-enter security was no big deal, considering that I happily did it many times to pick up/drop off loved ones at the gate.