Server fees? I doubt it. Then why would other major torrent sites (
http://www.torrentbits.org/) go down at exactly the same time?
I bet they were pressured by the MPAA/RIAA with a lawsuit. Of course, Suprnova had the law on their side, but a legal struggle with the likes of the MPAA/RIAA does not make much financial sense. They probably promised not to bring them to court if they quietly shut down the site with a bogus (or no) reason.
On the other hand, I thought all of their servers were located offshore and they couldn't be touched...
The other likely scenario is this is just clever publicity stunt: freak out the warez community for a week and then come to their rescue when they release their Exeem torrent client. :cynicalbastard