I was just wondering if there are ANY unique magic katanas that would make the proficiency worthwhile (besides Dak'kon's Zerth Blade, which is only in the collector's ed. unless I'm mistaken)... Or any especially nice longswords that can be found.

Heh. Yes, there are some nice katanas. Try the Celestial Fury +3. Lightning once p. day, blindness once p. day, stuns whenever it hits an opponent (save vs. spell), 5% chance of 20 electrical damage with each stroke. Very impressive. I'd even go as far as to say it's the best weapon in the game.

And like when and where do you get it?????

Whoops, I forgot to mention how you get it (spoiler). It's in the "Guarded Compound" in the temple district. You can get it right at the start of the game as long as you have a mage with level four spells. Go in and be treated to a little cut sequence, then go up. You'll have a fight with a bunch of seemingly tough summons, a few of which can be easily dispatched with a cloudkill. Get your fighter to take care of the Nishruu quickly as possible, kill the Djinni and then focus everything on the daemon (quite an easy battle if you just cast protection of evil 10'). After they're dead, go up the stairs and immediately cast emotion in the center of the group. If the fighter with the celestial fury didn't go down, hold person, stun, do whatever you can do stop him from attacking you, or he'll make short work of your whole party. Once he's out, everyone else is pretty easy, and you get some very nice stuff on top of your new katana.