WordPress MU

I was obviously originally running WPMU 1.1 with the WP-CACHE plugin

I've moved to 1.1.1 and tried the WP-internal cache
ENABLE_CACHE'true in config

How can I prove its working? Doesn't seem to mark the HTML source like WP-CACHE.

You have to make a cache subdirectory under wp-content and set it to 777. After you do that, you should see it start to fill up.

Not entirely relevant, but 777 is generally a bad idea.

Alternatively, if your server supports it, configure SuexecUserGroup username usergroup on a vhost-basis, set the directory in question (cache) to be owned by that username, and chmod it 700.

(I know this is kind of OT, but "make wp-content/cache and make it writable", thanks now I learned how to use ENABLE_CACHE, make ANYTHING mode 777 on a server.. you got to be shitting me)

Agreed but how many of us here have the ability to assign ownership of a directory? I think the basic user here is on CPanel or something simuliar and can't even see ownership.