Okay. Maybe it just takes a while for the cache to build – now I'm seeing cached pages (or at least directories for them) in wp-content/wfcache.
More to the point of the OP however, those cached pages seem to be using the mapped domains, and not the subdomain pages. For example, I see a directory for a page: acwprogram.com_staff, when the orginal, un-domain-mapped url for that page is: test3.bethannon.net/staff.
The site does seem more zippy, but the overall pingdom load speeds didn't improve dramatically. (Maybe it's a placebo effect?)
Mark, it seems like my initial tests on multisite presented no issues. Anything else I should look for or be aware of before I deploy this on a production multisite network?