Between Ghostcrawler's blog post and Zarhym's additional notes on the forums yesterday, we now know a lot more about how account-wide achievements are going to work. No preamble this week, folks -- let's get right to it. A lot of questions were answered, but there are still a few unknowns.
Question: Will all of my achievement titles be accessible from every toon?
Answer: Probably, but there may be restrictions on their use.
I've been looking to farm some nice titles out to alts that have done absolutely nothing to deserve them (who isn't?), and what we do know is that Blizzard's looking for a way to do this. From how GC's written about it, I'm wondering if this might actualy go live after Mists of Pandaria has already shipped, because Blizzard's talked about the technical limitations previously, and it's apparently still at work on it.
We also don't know when these titles will become universally accessible. For example, your level 1 monk may not get access to Kingslayer until level 80 or Defender of a Shattered World until level 85, which is sensible insofar as my level 1 monk was not in much of a position to save anyone with Jab as the full extent of her repertoire.
For now, put this firmly in the category of "Probably going to happen, but don't expect a firm date."