Which CL implementation is this?
Lispworks 5.1 on XP.
Hm, I haven't tested on Lispworks in a long time. What I suspect is happening is that it has a slightly dumb redefinition check -- what's happening is that defmethod is being called from a function (for every finalised dao class), with an (eql class) specialiser where class is a lexically scoped variable. Most lisps seem to realise that class is bound to something new every time the function is called, and don't complain.
This should be harmless. If it is causing problems (warnings at inappropriate times), could you find the Lispworks-specific magic-wrapper that can disable these warnings (if there is such a thing) and provide a patch where the relevant defmethod calls (function build-dao-methods in postmodern/table.lisp) get conditionally wrapped by such a thing on Lispworks?
Best, Marijn