The pattern matches for everything except maintenance.html and sends the request to maintenance.html. That is important, because at first I tried to redirect all requests by matching on ^/(.*) but that got into an endless loop, since it would redirect request to maint page which redirected to the maint page which redirected to, well you get the idea.
Here is my rule:
#RewriteRule !^/maintenance.html$ /maintenance.html [R,L]
Save file, cd ../bin and sudo ./apachectl restart and presto! All requests go to maintenance.html instead of the intended destination. Comment out those 2 lines, restart, and all normal.
Links of interest:
FOLLOWUP - 3/29
My maintenance redirect wasn't working for requests already coming in over SSL. I had to add the rule to the secure virtual host config section as well for it to work. Special thanks to "shimmyshack" on the alt.apache.configuration newsgroup for pointing me in the right direction.