I've decided to take jmp.li down. The code behind it is open source, and I'm open to offers for the domain name. Existing short URLs will continue to resolve for a while but I don't plan to do any more maintenance on the site.
Why? It's mostly being used for spam and porn URLs, and that's not a service I want to provide. I have no interest in marketing the service to compete with the likes of tinyurl.com, bit.ly and the other established sites.