I have a 3G iPhone and an iPad... here is a lame work around, but it does work....
Open Dropbox on the iPhone. Open the book you want to send to Stanza. Tap the link symbol in the bottom left of Dropbox. Tap "copy link to clipboard". Close Dropbox
Now open Stanza. Tap "get books" on the bottom menu. Tap "edit" and then "add book source". In the dialogue just give it a name (perhaps of the book) and then past the URL you got from dropbox. Tap and and then done. Now tap your new source, and you will be prompted to download the book.
Like I said, lame, but it will work
