Bookeen's specs say "Portrait and landscape mode"
http://bookeen.com/specs/ebook-specs.aspx
The Linux Devices reviews says "Enhanced layout quality (header, footer, margin)"
http://www.linuxdevices.com/news/NS4810239944.html
So I'm hoping that means rotated letter/A4-size PDFs will work, and work really well. I have a number of DRM-free PDFs, and I can see myself buy even lengthier books in this format if I was using an E Ink screen.
Some sort of conversion before hand is a nice backup measure. Though I'd want to know that it works before purchasing the device. Looking forward to the reviews!