the easiest solution (i think) is to rotate the pdf in acrobat pro
pages panel > options > rotate pages
Your book in landscape will be bound on the 8.5 inch short side.
However, if the ONLY option is Portrait, it seems they will bind it on the 11 inch long side, which is probably not what you want.
You can't assume they will realize your book is different and bind accordingly. Their system may just be set up to only bind the tall side.
You need to contact this on-demand publishing site before you do anything else. Explain what you have and what you expect.
There are issues that need to be clarified and clearly stated before you order. Also, the price may change.
I think Lilia is probably right, but there's a really old script you can use to rotate pages into a new file (it was designed for doing calendars before we had Rotate Spreads). http://forums.adobe.com/message/1308215
You may well need to put this into a "Version 3.0 Scripts" (without quotes) subfolder of the scripts panel folder to make it work in a recent version of ID.