A downside to this approach is not having an easy way to purchase books without a computer. iBooks has a great checkout experience and the only way O'Reilly could have something similar is by coughing up 30%. Tech savvy users can figure installing something like GoodReader and then downloading the PDF in MobileSafari and sending it over to GoodReader, but it's a UX nightmare. O'Reilly at least has technical users, other publishers are even more screwed if they want to go non-DRM.
Given that O'Reilly books are of technical nature, and you can't actually code on an iPad (unless you're using SSH/VPN), this is a perfect fit for the O'Reilly... you'll need a computer anyway.
Furthermore, I'm never on vacation and think, Oh, I'd love t read up on RoR... I usually already have the book.