How about the engineering specialization BoP pet? What would this require of me... hrm... level engineering up high enough to choose a specialization, then drop and renew the membership a few dozen times and cross my fingers the pattern arrives in my mailbox. Well... out of the 2 to 300 gold I've dropped in renewing on my gnome for the Gnomish Engineering pet (and still haven't had it mailed to me), it might be easier to do this than grind more whelps or hope for another crocolisk.
I planned it out. I located a decent engineering guide and calculated out how much of each ore I would need. A guildie sent me several stacks of leftover ore and bars he had floating around. I started up a Death Knight I had planned and once I got her out of the starter zone, sent her over to Elwynn and started making circuits picking up ores. Once I figured I had enough ores, I mailed everything over to my main, and late one night, took a deep breath and started.
Let's start with making room to take engineering...

Skinning is easier to drop and level, right? Ugh. Been skinning for 80 levels! This is gonna hurt.

Well, dropped. Aww crap. Forgot about that Master of Anatomy skill! Well hopefully this will be fast and I'll get my crit bonus back. Let's learn some engineering!

Time to get started! Oy, what crap I make!

So much junk. So many low level crafting items. I like crafting skills, but the excitement isn't quite the same when you're powerleveling through junk on a high level toon!
I'll save you all the "action shots" of grinding up to 205. Suffice to say, I ran out of mats and had to flip back to my DK to farm more plus hit up the AH when I got tired of that.

Dinged 205! (Actually 207 because I wasn't paying attention and didn't get around to crafting the items early on that I needed to turn in for my Goblin Engineering specialization!)
Time to run to Gadgetzan! The decision was made to go Goblin Engineering since my warrior currently has Gnomish Engineering. I researched and queried the communities to see if it was at all possible to loophole the system, but it's pretty much guaranteed that you will never be able to get both BoP specialization pets on one character. I calculate my pet collection in two ways: total on one toon for achievements and total across three toons (for sheer collection purposes). So as long as I get both pets, I satisfy my total collection requirement.

Back in Gadgetzan. Here dood, have an exploding sheep or five. Boy does this make me glad I fixed my goblin rep. Sooo much would still be closed to me. I love me some pirate booty but arrrr... pets and recipes are awesome too!
Next step, renew that card. Over and over again. At 2 gold a pop, it can add up quickly. My luck's been crappy on my warrior so I cringed while I dropped nearly 200 gold in renews.

Which meant I got a lot of presents in the mail. Wait... what's that you see at the bottom??? A PET PATTERN!

So much work and it came in on the first attempt! Woo hoo! On a totally ironic note, I actually got *two* bombling patterns. That made me really annoyed. WHERE'S MAH SMOKEY NOW??

D'aww! Isn't he cute??
He makes little rattle noises when he walks around! I actually didn't have much interest in having this on my list of random pets to pop out, but the noises he makes and the way he runs when I'm on a mount were rather cute. Plus, it's a pretty rare pet and draws questions, especially since I don't have engineering. So he stayed on the list of potential randoms to pop out whenever I don't have a pet out (go go Livestock addon!)

And here goes Engineering and back to Skinning. Yowch.
I considered keeping engineering for the epic goggles at the high end, but decided against it... for starters, I really like the crit bonus for Skinning. Then there was the pain in the ass part of not being able to supply leathers for my leatherworking and while there's currently nothing I need to craft and am at max, I still get guildies asking for gear for their alts and leg kits for their mains.
It took about a week to get my skinning back up to max but I didn't focus on grinding it like crazy except for the first two days since I needed to be in Azeroth for it. The worm cave by the Hodir dailies was great for getting the last 20 points or so.
Now the question is... was it worth it? You tell me...

What a place to bring a baby deer!
ETA: OH HAY INTERNETS!
(for the wowinsider readers: I did get this pet on the day of the new achievement system. I did the engineering grinding a week before the patch and got the deer the day of it. I just took my time posting the pics.)