The first one's fairly harmless. I too wasn't sure how to get out of a battleground until a friend informed me of the /afk trick. You're not allowed to go AFK in a battleground, so just purposely go /afk and Blue will boot you (with a deserter buff that wears off without consequence shortly).
The other exploit is a little more troublesome. Apparently, when a raid leader joins a battleground "as group," members of the group can click the button to join in without the group leader doing so. Which means those members can check the player screen to see if they're up against another premade (by noting if all the other faction's players are from a single server), and if they are, the group leader, still out of BG, can then join another BG (say AV if the original players are in WSG), and then join back to WSG when another instance of it opens up. Understand all that? Basically they're using the fact that a group leader can stay out of a BG to constantly enter and reenter battlegrounds until they're matched up with a nonpremade. Then it's honor farming time, and 3 captures later the process starts all over.
Now, I'd hate to be the tattletale on this, but I'd assume Blizzard knows about both of these "sploitz" (the first one isn't really an exploit in the strictest sense, but it is using an ingame function for a benefit it's not meant to confer). What players have done here is construct their own fixes for Blizzard's "bugs"-- there is no PVP matching system yet, so they've created their own (even though this system matches for the win, not for fairness). And there is no easy command out of the BGs (even /leavebg would be great and easy to implement), so players are using /afk.
Which begs the question: does Blizzard care about the battlegrounds as they are now? Or are they too focused on expansion PVP to fix these "flaws"? And if they are too focused, is that a bad thing? We already know the honor system will be scrapped, as well as the current LFG and most of the PVP system. Does Blizzard have a responsibility to fix what we have, or is it OK that this musical chairs of premades continues until expansion release?