The latest m0n0wall, 1.2b7 adds a local user database for managing the 
captive portal access, along with some nice logging for authenticated 
access (only).

Its possible to add captive portal logging even for anonymous portals 
"read this, click here to continue" by doing the following:

1) in the Captive Portal -> Users section
2) Add a user with:
username: guest
password: guest
Full name: guest
and click Save
3) modify your captive portal page so the form section reads:

<form method="post" action="$PORTAL_ACTION$">
    <input name="auth_user" type="hidden" value="guest">
    <input name="auth_pass" type="hidden" value="guest">
    <input name="rederulr" type="hidden" value="$PORTAL_REDIRURL$">

    <input name="accept" type="submit" value="Continue">
</form>

(You can change the accept input tag to whatever you want instead of 
"Continue").

4) Go to Captive Portal and
a) change Autentication to "Local user manager" (from "No authentication)
b) provide the modified page filename
and click Save.

This will have the effect of logging on your anonymous users as guest, 
which menas their info will be entered into the log (MAC, date/time).

Hope this helps someone...

cheers, michael

PS: CHris, I hereby release this into the public domain, in case you 
want to add this to the docs/faq.