I'm in IT, but as a "software support specialist" (an "administrator" over some of our third-party software). From my perspective, it is a matter of resources (programmer time) and priorities.
This is a task that one wants to get right, and not only does it affect a fungible set of customers, the set which tend to be very picky about their money, but it also affects numbers that are reported to the IRS (both as fees that customers pay and losses that Prosper claims), and the IRS is also very picky and carries a very big stick. So this is something that, if you do it, you want to do it right, i.e., spend more time testing than you would spend coding--you aren't going to do it in just a day or in just a few days.
A conflicting priority is that there are some federally mandated and state-mandated changes that one may have to make, such as making sure 1099-INT and 1099-B forms, or their equivalents, meet the latest requirements, and they have to be completed, tested, and the real forms generated and distributed1 by the end of January. I don't know how it is with Prosper Marketplace, Inc., but, where I work, last month we have received updates from our administrative software vendor for 1098-T, 1099-MISC and W-2 forms specifically for Tax Year 2007, which means, if we were doing this in-house, our programmer/analysts would be working on those changes instead of waiting for the vendor to ship us updates. As it is, these updates get priority over almost everything (except System Down), so a non-trivial refund process would be pushed out to after tax-related issues are taken care of.
Note1: Year-end tax forms (1099-INT, 1099-B, W-2) generally have to be "distributed" by January 31. In this context, "distributed" means in the mail (postmarked January 31 or earlier; interoffice mail doesn't count) or electronic notification sent out if the forms will be viewed on the web instead of paper copies mailed out.
So I can easily see four weeks taken up with IRS-related tasks, and, after that is taken care of, spend four intense weeks for refund-related tasks. That sounds like a good reason for the delivery date to be placed two months out.