Haskell is currently the main focus of the EclipseFP project. You'll find more general information on the EclipseFP 1 and EclipseFP 2 pages.
Complete instructions to download and build Eclipse FP2 can be found here.
For developments mainly concerning the integration of Scion, see Thomas ten Cate's EclipseFP GSoC '09 blog.
There is an article that describes the approach taken to parsing Haskell code in EclipseFP version 1: EclipseFP Haskell Parser Structure by Thiago Arrais (thanks Thiago!). In the EclipseFP 2 branch, this parser structure is no longer used.
The general topic of implementing part of an Eclipse plugin in Haskell was described in a paper for the Eclipse Languages Symposium. The pdf for that paper is available here.