Maybe someone has already thought about this angle so forgive me for thinking out loud...
To purchase bitcoin with paypal, a money transfer can be made, and as long as you get the user to perform some sort of double confirmation of a sending address for X amount of bitcoin. Also emailing a reciept, then you could effectively, and publicly show proof of delivery by showing the block that the transfer happened in right?
Would paypal not also accept a tickbox saying you agree to various terms and conditions about reversals?
That dog won't hunt.
It matters not if you can prove the bitcoins were delivered.
PayPal simply expressly prohibits using PayPal for digital currency transactions. Period. (Well, VirWoX found some type of exception) They may not be able to always tell that a transaction was for Bitcoin but when they do it is brutal. Nearly every person that accepted PayPal in exchange for bitcoin has either had their account closed, funds frozen, etc.
"I got suspended for selling paperclips with free bitcoins 2 days ago, and yes i sent the damn clips with a tracking number. "
- http://bitcointalk.org/index.php?topic=67137.40
- http://www.bitcoinmoney.com/post/5086167006