database structure - general

[eluser]jedd[/eluser]
You are right to be wary of de-normalising your database.

Stick with storing usernames in your user table, and storing user ID's in your message table.

also, anyone have any idea how to say "last post: 15 minutes ago" like in these codeigniter forums, rather than "last post: 9:30". i think the former is a lot easier on the brain...

How far have you got with this one?

A function that takes a given DATETIME stamp, and calculates the delta with current time, then gives a 'fuzzy' string back - it's just a matter of calculating what different time periods should generate what response. Possibly a large switch or a series of if/elseif's I'd imagine.