In the campaigns I've played using 3.x I've noticed that only really combat oriented mages tended to wear armour.
They weren't necessarily multi-classed, but it was usually the characters who didn't mind getting their hands dirty. (and usually the guys in armour don't use their feats to get still spell, but rather either spell augmenting feats - like maximize spell, or proficiency feats)
We've had a pure wizard wear a chain shirt, I've played a paladin/sorceror who wore a breastplate, and we've had cleric/wizards wear leather or studded leather armour.
Mind you I think that it depended entirely on the character concept and whether this would work (most wizardly types in the campaigns I've played tend towards the more "gandalfy" type magic users - robes and big hats)
However, in saying that... I've found that its generally multi-class wizards and sorcerors or bards that tend to use armour. (while the single class wizards and sorcerors tend to rely more on the armour spell)
I still remember the paladin/sorceror. And in hindsight, I should have concentrated on spells that boosted ability scores, rather I concentrated on spells that would assist with the character concept (he was a holy warrior for the LN god of magic in that campaign) So he ended up getting spells such as identify, and other fairly subtle spells (he thought of these as gifts from his god - I suppose it was all about flavour back then)
Then again, I don't suppose most people suspect a sorceror to walk around with a greatsword.