What is "Scala Presentation Compiler"?

The Scala Presentation Compiler is provided as part of Scala so that IDEs can have access to the intermediate information that doesn't make it into the final compiled output.

A quick googling gives this. Odersky also discussed it in this (very interesting) talk about what's coming in Scala 2.10 (at about 22 minutes in).

It's a compiler that provides the hooks necessary to work well to support the interactive features of an IDE, like color-coding, autocompletion, etc. One of the technical challenges is efficiently staying in sync with what's been edited. See this talk.