Safari Books was one of the first web sites I tried. Since there's no Flash, books like the Head First Series are Flash only, no HTML view, then those won't render. It seems some books are "scanned" in and available as images/Flash only.
But it just occurred to me to try the mobile version - m.safaribooksonline.com.
I don't have one, but live near a store. I'll have to try that - if it senses your hardware isn't mobile, it won't work. I wonder what the it'll reject the iPad.
Books that don't render as html look fine on the iPhone in the mobile version.
Update: Ah, looks like Safari Books, the mobile version does work on the iPad. See the Apr 2nd Tweet:
http://twitter.com/Safaribooks