(disclaimer - I'm the CEO of Atlassian)
Developers are religious. Whether it is vi vs emacs, or IDEA vs Eclipse or Mac vs Linux. In September we had a huge debate internally (hundreds of comments long) on functional vs non-functional programming.
We know that no-one wins in these religious debates.
We've found that developers are equally religious regarding their DVCS. Most frequently it is whatever DVCS you started with is the one that you're passionate about.
We've found that most companies of any size have both Git and Mercurial lurking somewhere in their development ranks. Previously if they wanted to host their code in the cloud (and who wouldn't), they would have had to choose two different hosting providers. As a developer, this was a pain as you never knew where your code was.
Now that Bitbucket supports both, you no longer have to choose. We think this is a huge win! Companies can store all their code in one place. And with the per-user pricing, it is a very fair pricing model (don't have to think about whether a fork is going to cost you or not!).