Yeah, there are currently a ton of great ebook apps like Kindle and other ones for viewing documents like PDFs, and I'm sure an iPad optimised version is coming soon from the majority of them. I don't think we need to worry about iBooks DRM. The only real problem is the software/app issue in my opinion, but jailbreaking somewhat solves that for people tech-savvy enough to do it.
Stanza is great, there is also GoodReader for PDF's, Wattpad, Amazon's Kindle app and the Barnes and Noble reader among others. Some of them have their own DRM, but there is definitely still choice.
Since the iPad can use existing iPhone apps, and other e-readers and dedicated eBook apps abound in the App Store, there will definitely be ways to buy books from other sources, or load unrestricted RTFs/PDFs/etc. I wouldn't put it past Apple to specifically block them, but I don't think they will.
I don't know why anyone expects they will be allowed to do that. Music is a form of digital IP that had already descended into lawlessness by the time the iPod came along. Movies (because until recently they were too big to download) and eBooks (because until recently there was no convenient way to read them) can still be "controlled" (controlled meaning continue to generate significant revenue for incumbents). Between Kindle and iPad, in 5 years most normal people will have a benchmark in their head that an ebook is "supposed" to cost ~$10 and they will be ok with it. Letting anyone load eBooks onto the iPad risks putting the publishing industry where the music industry is now.
The major difference is that there are many books that people will want to read that are in the public domain. Also, as far as I know, their book-reading competitors (Kindle, Nook) allow un-encumbered ebooks.
I'm not really sure why Apple putting DRM on media they sell is an issue as long as they also allow non-DRM'd stuff to work just as well.