: From my limited knowledge:
:
: Remember that macros are functions that run during compilation.
:
: When you call:
:
: (define-foo-class bar
:        ((baz  :initarg :baz  :accessor baz)
:         (quux :initarg :quux :accessor quux)))
:
: the *compiler* has to call the macro function and therefore has to call
: your id function.
:
: So in order to make sure that the compiler knows id's definition,
: wrapping id into a eval-when form is no mere workaround but standard
: technique.

Thanks, Stefan.

I see that FAQ 3-7 also addresses the issue:

http://www.cs.cmu.edu/Groups/AI/html/faqs/lang/lisp/part3/faq-doc-8.html

I was confused because in chapter 24 of PCL, Seibel defines similar
macros that call functions, and all the code is ASDF-loadable.

The difference that I missed is he defines the macros in one package and
expands them in others after the whole supporting cast has been loaded.