I have 3 installs of WP 3.0. Two servers running PHP5 & MYSQL5. One running PHP4 and MYSQL4.
In all three cases, when I go to SuperAdmin>Option, and try to uncheck the option to "Allow site administrators to add new users ...", it won't save to the database. If I manually change the value in the database, everything is good. Then WP will allow me to use the Network Options page to check the box and everything saves correctly. It just won't allow unchecking and saving.
I've removed all plugins and custom themes, and still not working. So it doesn't appear to be related to those.