I was intensively researching headphones 'bout three weeks ago when I tumbled upon this great thread.
Since then I bought my cans. I decided to go with the Sennheiser HD595. I know that this is not the cheapest out there - I bought it for 170 from "the Bay", new.
I based my purchasing decision on the followings:
Headphone technology is matured enough and is not expected to change radically. Just like with speakers, it is reasonable to buy a better set for a little more money – it won't get outdated quick unlike some other electronics.
I wanted a solid, over-the-ear set with open-air design. It is true that hey make you look just like Princess Leila, but they tend to be very comfortable (no sweaty ears) and they sound fabulous thanks to bigger drivers. I don't plan on wearing them outside my house anyway…
Sennheisers are constantly on the top. The 595 is a great buy in particular – it is a newer design with extremely low impedance, so even today's small audio devices can drive it with their weak amps. (I tried two other headphones on the underpowered "headphone out" connection of my receiver, and they pretty much sounded like cr@p with a lot of background noise. So I was a little concerned when first trying out the 595, and it sounded effortless and super clean even at lower volumes. Bingo!)
(Word of caution – there are two versions out there, the one I am referring to has a lower, 50-Ohm impedance – don't buy the 120-Ohm version – you might have to purchase a separate headphone amp for that one, and those are a little on the expensive side!)
There are other great solutions out there for differing needs, but I can recommend this one whole-heartedly.