Principles of random effect analyses
These analyses are also called second level analyses, and concentrate on the variation of activation between subjects in deciding the significance levels for SPMs.
For a comprehensive description of this problem in SPM, see the random effects chapter of the Human Brain Function book - second edition.
See also the RFX section of the FIL/SPM bibliography.
There is a nice random effects discussion on the Wisconsin site
A useful set of powerpoint slides on random effects, by John Aston, at the MRC Cyclotron Unit.
Jesper Andersson's nice talk on Mixed effects models and variance components.