That's a misunderstanding, guys. No need for a desktop app, jailbreak or anything like that. Simply email the PDF file to your iPad or iPhone (to your regular email account), and you can open the PDF file in the Kindle app right from the email attachment.
That's only one option, but there are others equally easy. Such as loading the PDF file in the Kindle app from your Dropbox or SugarSync app.
(However, I don't recommend using the Kindle app for reading PDF files, or indeed any books. There are much better iOS apps for handling PDFs -- the most prestigious of them is GoodReader, but now I like PDF Expert even better.)