Jonathan Eisen has got the right idea, listing the entire members' directory of the AAP and calling on academics to consider a boycott if those entities will not at least request dissociation from the PRISM program (as Rockefeller University Press has done) or its discontinuation. You can also read the members' list on the AAP site, and Peter Suber points out that we should pay particular attention to their Professional and Scholarly Publishing division:

I suspect that AAP/PSP did not consult its members before launching PRISM. But in any case the members should know that the launch of PRISM tarnishes them, alienates authors, readers, and referees, and, if successful, will only harm science by entrenching rather than removing access barriers to the results of publicly-funded research.

Peter is commenting there in response to someone else who has got the right idea, Peter Murray-Rust, who (as a Cambridge faculty member) has written to Cambridge University Press; his letter is an excellent example of what everyone should do who has any connection, professional or personal, with any of the AAP/PSP member companies, so I quote it here in full:

Open Letter to Stephen Bourne, Chief Executive Cambridge University Press

Dear Stephen Bourne,

I am writing as an individual member of staff in the University (heavily
engaged in developing new approaches to scientific scholarly publishing) to
ask about CUP's involvement with the recently launched PRISM initiative
from the AAP (http://www.prismcoalition.org/). This initiative is an
undisguised coalition to discredit Open Access publishing and its launch a
few days ago has generated universal dismay and anger in many quarters
including several outside mainstream publishing. The press release was
reported in full by Peter Suber on his Open Access News blog
(http://feeds.feedburner.com/~r/earlham/dGCQ/~3/147374721/2007_08_19_fosblogarchive.html)
where he has objectively answered and dismissed the basis of PRISM and its
methods. As an example of the language of PRISM it implies that publishing
in Open Access journals (as I do on occasions) is "junk science". There is
much more from PRISM which is both deliberately factually incorrect and
misleading and I cannot see how a reputable scholarly organisation such as
CUP could be associated with it. Indeed at least one similar publisher
(Rockefeller University Press
http://feeds.feedburner.com/~r/earlham/dGCQ/~3/150207794/2007_08_26_fosblogarchive.html)
writes:

"I am writing to request that a disclaimer be placed on the PRISM website
indicating that the views presented on the site do not necessarily reflect
those of all members of the AAP. We at the Rockefeller University Press
strongly disagree with the spin that has been placed on the issue of open
access by PRISM." [rest of letter omitted here]

The purpose of my letter is simply to request factual information from CUP
about its involvement with PRISM. Since PRISM itself has not reacted to any
of the recent comment I can simply speculate that not all members of the
AAP (perhaps including yourselves) were consulted before PRISM made its
press release and new site. In particular it is unclear whether PRISM is de
facto composed of all the members of the AAP or whether it uses their
unsought goodwill to reinforce the apparent strength of the PRISM
organization.

This mail is an Open Letter (posted on my blog,
http://wwmm.ch.cam.ac.uk/blogs/murrayrust) and I would intend to publish
your reply in toto and unedited since your position (and those of similar
publishers) is of great public interest). If there is anything you would
not wish to be published, please indicate. Alternatively you may leave a
comment on the blog itself. (My blog itself, though strongly advocating
Open Access and particularly Open Data, attempts to be fair and accurate).

Thanks in advance

Peter Murray-Rust

This letter hits every necessary nail squarely on the head:

• be polite
• make clear the nature of your connection with the publisher to whom you are writing
  
• keep the background brief and be sure to point to Peter Suber's rebuttal
• explicitly request a specific response: did Publisher X know about PRISM, and does Publisher X support PRISM?
• suggest that Publisher X should publicly distance themselves as RUP has done
• if at all possible, do all of this in public: an open letter, on a blog
  

I don't know whether I have any direct connection with any AAP/PSP member companies, although I could certainly write to publishers of journals in which I have published papers. In a later entry I will dig through the list and try to find likely recipients of such letters -- for which Peter Murray-Rust has provided such a splendid template.

Update: in comments on Jonathan's post, CSHL Press has repudiated PRISM. Good for them, and I hope they will make a formal public statement to the same effect -- for instance, on their website.