The image pretty much says it all; Prinny from the Disgaea and Prinny games is wearing the "Curly Brace" hat from Cave Story, and that gun and that scarf look quite Cave Story-styled as well. Other that, we have no idea what's going on here, and you know what means -- time for baseless speculation!
With the 3DS and the NGP coming soon, it's possible that this image is a tease for a new Cave Story and/or Prinny project for one of those consoles. It could also be something for XBLA and/or PSN, as fans have been quite vocal about wanting Cave Story on one or both of those two platforms. Then there's the fact that hackers have had Cave Story running on the PSP for years now, and NIS is quite the prolific PSP publisher, which has me thinking that an official retail PSP port of Cave Story could be incoming.
I've contacted Nicalis (Cave Story's publisher) and NIS (Disgaea's publisher) for comment, but as you might expect, both companies have declined to release any additional info at this time, though Nicalis has promised me that whatever is going on here will be awesome, and that they'll tell me more as soon as they can.
I guess it goes without saying that I'm excited as f*ck.