Yes, there is. Basically, all of the sites submit themselves to a directory, and you can start browsing that day. I did post-UND last year, and started to submit things quite soon after. I do think that there was one site in a major metro that was posted the same day and UND and then no longer available that day. However, most sites had a less crazy turn around of some days/a couple of weeks. Yet, it is still quick: there are no real set rules on when places can make offers, etc. Thus, sites are really trying to get qualified applicants before other sites, so application to interview to offer can go pretty quick (I think mine was ~ 1 week from application to offer; around 2 weeks after UND), but I had several offers (around 3 days from application to offer) before that that I turned down due to geography.
There were lots of really terrific sites last year. I think I definitely "matched up" and am getting some solid training. I think there are less applicants at this stage, so it seemed like sites were competing more for me. So, even if you don't get your desired site the first go around, there can be some really terrific spots after UND (e.g., many well-regarded VAs, child sites, etc. were in the directory last year).