Thanks for the reminder,
Here is what I've got at the moment, it is a lot more "bloated" than any other Payment method as I am planning to eventually include support for subscriptions and buy now buttons as a Field. I've also added a lot of formatting checks by way of setters/getters for each PayPal HTML Variable following the PayPal specs so errors are easier to catch and display via PHP.
Module is currently based off PayPals HTML Variables Reference (Updated on June 1, 2009) so it may pay to check that nothing major has changed between now and then.
Let me know if you need some help getting it to work.