Ah necro'd threads.. Sometimes, they bring back stuff I've never seen:
Another little-known tip--F1, F2, and F3 are direct shortcuts to the PIPboy tabs!
I actually want to add onto this:
You can also close your Pip-Boy using the same F key as your current tab. I.E. If you're in the items section of your Pip-Boy, press F2 to close it.
So in actual fact, there's four keys to close your pip-boy as well as four to open them, it just depends on which section you have on display.