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. See Andrew Holmes' [http://www.fil.ion.ucl.ac.uk/spm/RFXposter.pdf poster on random effects] and [http://www.fil.ion.ucl.ac.uk/spm/RFXabstract.pdf abstract] (both in pdf format), and [http://www.fil.ion.ucl.ac.uk/spm/spm96.html some links to SPM mailing messages on random effects].
Darren Gitelman has also put together a very helpful [http://www.brain.nwu.edu/fmri/spm/ranfx.html summary and collation of SPM mailing list answers on random effects] with some links to papers on random effects.
There is a nice [http://psyphz.psych.wisc.edu/~oakes/spm/spm_random_effects.html random effects discussion on the Wisconsin site]
See also a useful set of [http://www.cu.mrc.ac.uk/~jaston/mrcjc/sld001.htm powerpoint slides on random effects], by John Aston, at the MRC Cyclotron Unit.