I've been using memcached for some time on phpMyAdmin wiki and it seemed to help. But recently the performance went down. I didn't have time to investigate it till today and it turns out to be caused by memcached. When I switch it off, page load time goes from 16 seconds to 0.25 seconds.
I tried to search for some similar problems on the net, but I didn't succeed. So for now wiki runs without memcached and I will look at other possibilities of caching later.
Update: If you're interested about versions, it is MediaWiki: 1.7.1, PHP: 5.2.3-1 (cgi-fcgi), MySQL: 5.0.41-Debian_2-log and I have enabled persistent connections to MySQL.