I've always raided as Frost, so maybe this is a silly question. What's the explanation for partial resists at odd values (not 25, 50, or 75)? A Fire Mage told me about seeing lines like "Your Fireball hits for 600 (400 resisted)" or some such.

only thing i can think of, if the target of the spell has some kind of resist/dmg reduce-ability (priest, 1.11 shaman for example)
maybe it shows the damage prevented this way as 'resited'
could explain some odd values

AFAIK some damage reduction/enhancement are applied after resistance.

Per example a warrior in defensive stance resisting 50% of a 1k bolt may see "450 (500 resisted)".

Usually it's due to a crit. I don't rememer exactly how it works as I haven't been fire in almost a year, but when a crit is also partially resisted it doesn't work like you would think. If you were supposed to crit for 1500 on a fireball and get 50% resisted, it wouldn't do 750 damage. I believe the resist is factored before the crit. So like, a hit would have been 1k, and it was 50% resisted, it would take the "hit" down to 500 damage, then add the crit damage of 500 more on top of that, giving it a total damage of 1,000, and a combat log message of "Your fireball crits xx for 1,000(500 resisted)."

Hmm, that would agree exactly with the apparent 40% resists.

I can confirm what is said above. Partial resists are calculated before anything else, so sometimes it looks like you didn't get a 25/50/75% resist when in actuality, you did.