i don't know! yet. but we may find out shortly.
wowinsider says that it will be murkablo, the diablo 3 themed murloc pet that was datamined in patch 4.2, BUT blizzard has not confirmed this... yet.
with the announcement of diablo 3 testing and the new d3 ah system, it's likely that this game will be released "soon" or at the very least announced at this year's blizzcon. with that in mind, it's not unlikely that blizzard would hand out d3 themed redeemable pets as part of this year's blizzcon goodie bag.
on the other hand, murkablo could be part of the collector's edition special gift (like mini thor was for sc2's collector's edition).
either way, the only sure thing right now is that blizzard should be announcing what this year's in-game redeemable item is soon. on top of the diablo 3 announcements, they've opened up the 2011 blizzcon virtual ticket sale as well.
so keep your eyes peeled for another announcement soon! *crosses fingers*