As announced by Monster Hunter International author Larry Correia here and here. Details such as publisher and system are sparse thus far, but apparently the current plan is to release the book via Kickstarter. That should be a sight to see, as in my somewhat limited experience Kickstarts (?) that have a premade fanbase tend to be quite spectacular. I hope Larry's got some good ideas for stretch goals . . .
In any event, being a big fan of the MHI series I'm quite excited about this news. I's tough to speculate on the details, though the author is a fan of Legend of the Five Rings (not that that probably means much). I could see this working in any number of systems, though GURPS is probably out due to their very similarly named, already extant series.
7 AUGUST EDIT: The Monster Hunter International Employee’s Handbook And Roleplaying Game will be produced by HERO Games for their Champions system. A Kickstarter campaign is scheduled for October.