I just discovered a way to sideload MOBI files to Kindle for iPad on a non-jailbroken iDevice.
Macroplant offers a
free app 'iPhone Explorer' which allows you to browse the contents of your connected iDevice.
When you open your iDevice with iPhone Explorer, there are two folders - Apps and Media.
Inside the Apps folder is a Kindle folder (assuming you've installed Kindle for iPad), and within you will find Documents/eBooks. For those of you playing at home, the full path is Apps/Kindle/Documents/eBooks.
I dropped a MOBI into that folder, and lo and behold, that MOBI can be opened in the Kindle for iPad app.
Update: iPhone Explorer occasionally crashes when dropping the file into the folder, and it appears that all sideloaded files share the same cover, but hey, you gotta start somewhere.
G