Thanks - I actually ended up doing that and the eReader books are in my Nook Tablet library now. I wish they could sync but you can't have everything.
I've since realized that the eReader.com app for the iPhone not only hasn't been updated for Retina display but has actually been removed from the App Store. Looks like it's going the way of Mobipocket. Sucks for early adopters, sigh. And I sure hope Nook survives longterm so I'm not in the same situation a few years from now!