You can be both factions on the same server - at least since I started playing (summer 2009).

Mix 'n Match as much as you like.

I was going to consider letting you not know, but then I just went "eh to heck with it."
Here's the lay-down regarding only one faction on realms:
This *used* to be the case, and only then on PvP and RPPVP realms. Now? You can roll as many of either faction on whatever realm you want (and even send mail with heirlooms to them, at that. Heck, in the future we'll be sharing pets and maybe even mounts account-wide...)--so no, the decision is not automatically selected for you.
As for Cross-Faction chat and etc, the Pandaren are closed off from the outside world (guilds, possibly mail and messaging any non-Pandarens) until they make their faction choice at level 10. By this notion, the Pandaren will likely not have a racial-specific race they can speak in--or if they do, their dialect will be untranslatable to other Pandaren of the opposite faction. There used to be Forsaken with common speak a long time ago, and it made problems, which is why we won't be getting it now.