Hi folks,
I'm working with a business that is preparing a quote for a potential new client who wants to use PaySimple (http://www.paysimple.com/) as their payment gateway (they have an existing site and relationship; we want to present a Drupal 6/Ubercart 2 option to replace it to meet their expanded needs).
PaySimple isn't currently supported by UberCart, but appears to be a site where all payment transaction processing takes place at the remote side similar to how Payflow works.
I have no idea if this can be incorporated within an UberCart framework (i.e., send purchase info to their site, pay, return the transaction info to Ubercart, close order) but I have no reason to suspect it can't be. May even be possible by replicating and ravaging chunks of the PayFlow module...?
My biz cohort may decide the bounty route better than pushing the potential client to alternatives, depending on the heavy-lifting / estimated bounty / timeline.
Thoughts...?

There doesn't appear to be any documentation on their website... all I could see is this page: http://www.paysimple.com/partners/software-integration-partners.html
You really need the developer/API documentation to understand what is involved, but all gateways provide some form of API that you can integrate with, the question is what features do they provide via their API's and what features do the need?