This server runs Debian Linux, and I've been running version 6 (Squeeze) for a long time. Squeeze was the first "LTS" release supported by Debian.
LTS stands for "Long Term Support" and is a tag used to denote longer than normal security updates and support. Well, we ran out of "long term" this week so I had to upgrade. Always a slightly worrying process on a live server but a quick hop-skip-and-a-dist-upgrade, it appears to have worked. Debian is one of the few computer operating systems that support OS upgrades without requiring a reinstall and it's a wonderful thing.
The only thing I have seen wrong afterwards so far is the calendar plugin for my blogging software (blosxom) : it broke with a Perl Storable byte-order issue. Easy to fix (at least temporarily) by disabling caching. Of course, I'll need to monitor it a bit more closely over the next day or so.
Hail Debian ;-)