I've spent the last 5-ish hours re-writing some of the code that works out whether you want to only see blog entries by a certain person tagged with a certain keyword/s, which means there shouldn't be more than about another 10 hours' work left to properly turn on the blog tagging system.
What's a tag, and why is it important? Tags are keywords assigned to a blog entry. This means that large, monolithic blogs with lots of entries about loads of different things can now be broken up using tags. For example, I could tag posts about the website with "UPSU.net", and tag random rubbish with a "ramblings" tab. This is important if we're going to make the UPSU blogs really attractive to people who want to write blogs here.
The way tags will actually do their job is through the URLs. Currently, a blog's URL is something like "upsu.net/blogs/alexh/YYYY/MM/DD". To only show blog entries in 2007 tagged with "noodle", the URL would be "upsu.net/blogs/alexh/noodle/2007" (and it works, too...), and you can still use the old style URLs.
Each entry can have more than one tag, so the same entry might show up in several searches - handy if a blog entry covers more than one topic.
Browsing tags will happen by showing the top few tags for each blog entry at the bottom of the entry, and also by showing the most-used ten-ish tags in a small tag cloud in the diary links bar, which will also give an overview of the blog's main topics.
Or this could all be an exercise in the introduction of a whole host of new security loopholes in the website... Uh oh...