Higher impedance cans (up to a point) are easier to drive as they require less current (V=I*R, raise R, and you need less current (I)). Nevertheless, they may not get sufficiently loud, which is as much about sensitivity as impedance. Nevertheless, all of your options here should work fine with an iPod, and most were designed for portable use. Amping may help low-impedance cans sound better (amps tend to have lower output-impedance) and high impedance cans sound louder. Nevertheless, this is a massive digression, nothing you've mentioned will have problems being driven by an iPod. Also, don't feel obligated to run out and buy an expensive amp. While it may improve some things modestly, most iPods have pretty decent amps and DACs.
Many feel the M50 overrated. I think there are probably better options at that price point. Namely, I'd wait for a sale and get the vmodas. Just FYI, the M80 recently was on sale at Radio Shack for $130, and is currently $140 on Amazon. If you're using your cans on the go, it might be relevant that aside from sounding great, probably among the best in this thread, the M80 is the first can to reach military specs for durability.
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I didn't realize the V-Moda M80 is selling for only $140. I would definitely go for that if I get the money. Thanks for the info on impedance, sensitivity, portable amps, etc. I guess I need to figure out my budget before I go any further... lol. The Panasonic HTF600, Creative Aurvana Live!, and V-Moda M80 all seem like great options to me, but the price differences are huge. I don't know if I'm willing to shell out more than $100 right now, so I'll probably go for the Panasonic or Creative headphones. But if I get a surprise jumbo birthday check in the mail I might reconsider.
All are good choices in their respective price ranges. You may want to start smaller. At $15, the Ksc75 are great. Another option is the KRK cans. I've never heard them, just their near field monitors, which were awesome . But, they're supposed to be great. All in all, I think you have some awesome options that will sound great with your iPod. A lot will depend on the signature you prefer. Don't feel pressured to spend tons. And while we're on value, Koss makes some awesome, great value stuff. The ksc75, portapros, and those all aluminum dj cans all have great reputations, though the last one may need an amp. Good luck!
One last thing, the CALs are amazing sounding for the price, but they're not super durable. If you have a preferred signature in mind, I may be able to help further