I was trying to install the Hpricot gem, but it wasn’t working. Ends up that you need to install the ruby1.8-dev package on Ubuntu…that’s where the mkmf file resides. I suppose it’s cuz Hpricot has things it needs to compile, and all those things are in the dev package. It makes sense now, but kind of annoying when you couldn’t have guessed. Good thing for Google.