Kobo just inked a deal with the world's most widely distributed smartphone maker. I don't have the numbers, but I'd bet that HTC have more devices in use than the 100m iOS devices quoted by Steve Jobs. Heck, they're so big that even I know who they are (and I don't cover smartphones).
I haven't found the press release yet so I'm just working off the top of my head. But I do have the announcement from the Kobo Blog:
Kobo today has partnered with HTC – which supports the Kobo global vision of enabling consumers to read any book, on any device – anytime.
Today, HTC announced the addition of e-reading to the HTC Sense experience – which includes a global eBookstore ‘Powered by’ Kobo! The HTC developed e-reading experience uses the Kobo Application Programming Interface (API) to deliver an integrated shopping experience.