MW has sprung a few leaks in the past. none were too serious and update patches were quick and effective IMHO, so like any other web application, keep your eyes peeled on the announce list and upgrade ASAP if there is a security update announced.
oh, and any sense of whether to go with betas or 'full releases' would be useful... As a 'proper' (NDA) beta-tester on some commercial products, I know that betas can sometimes go adrift when new things are implemented - and I have people using this wiki actively. So where is the balance with Mediawiki? are betas pretty solid - especially ones with security fixes - or are they 'much emphasis on "beta"'? (I think you get the idea...!)
much appreciated...