The Mailbox team, in a blog post announcing the news:
Today we’re thrilled to announce we’ve added Mailbox support for Yahoo, iCloud, me.com, and mac.com email accounts. This is a big step for us — we get more requests for Yahoo and iCloud support than for any other feature.
Since Mailbox first shipped, I've been one of those people clamoring for iCloud support. And it's now here.
The Mailbox guys were gracious enough to include me in the beta for this feature. I've replaced Mail on iOS with Mailbox as my main email client over the last month or so for testing, and the iCloud integration works great. Setup is dead simple, and I really like the app. Beautiful and easy to use.