Before the iPad was released, I noted how Amazon aggressively ramped Kindle promotions. The marketing seems to have shifted to platform / application: regardless of the device, Kindle can power your reading experience. Additionally, the same core messaging applies: Amazon's selection in unrivaled. As an iPad owner and iBooks user, this is resonates.
They marketed 420,000 titles on March 1 and less than two months later that number is 500,000+ - a 20% gain.