[StyleElements]

I use the QC15s for flying, but one IEM which has worked well for me on planes is the Klipsch X10. It's a little more expensive now (I bought them for $85 around Black Friday last), but they isolate plane noise really well for me with the dual flanges. It's probably the small size and the deep insertion. The bass also rumbles nicely, drowning out additional noise.

I'd only pick it up if you like a warm sounding headphone. For a single BA, the X10's bass is pretty strong - quite punchy and well-defined, more like a dynamic driver. I haven't heard any of the Ety line, but from what I've read the X10's sound signature would be totally different. I used them on my last flight (14 hours... yuck) and they worked a treat.
 
Good luck with that!
 
http://www.amazon.com/Klipsch-Image-X10-Noise-Isolating-Earphone/dp/B000WAHFBK

 

[Dr4Bob]

I also would also recommend the Etymotic IEMs
for passive noise cancelation. The triple
flange tips are very effective albeit somewhat
uncomfortable for some users.

 

[RIQUE]

Let me tell you I did the research for you about two weeks ago. I own a set of Sony MDR NC 11 that are past their prime. I decied to get new set after owning these nearly a decade.
A very important thing is that if you think that a high isolating IEM set will do the same job as a NC unit your are wrong. The tech behind the NC units is getting pretty darn good and so is the sound
the offer as IEMs.  I researched every sony, jvc, sennheiser...you name it in ear  NC headphone and after reading review after review I concluded that the AKG K391 had the best NC tech and also the best sound.  It has digital cancelling tech, the NC is defeatable, it has a rechargable battery and a mic in case you want to anwser the phone.  My set has not arrived yet but I hope to review them against my Sony and other high qualtiy IEMs so if you want to wait a tad...

 

[waimak]

Ok, so late to the party, but...
 
while I know they appear to be a love 'em or hate 'em headphone, I find my Senn HD380Pros to be good on flights.  They don't have the active noise cancelling, but do have good sound quality and isolation.  That said, a portable amp always helps, otherwise the DAP may have to be turned up a bit more than normal (well, mine does).

 

[CBus Audio]

Another vote for the HF-5 but I got the ACS custom tips with them and the comfort and isolation are phenomenal. With prices going down on the HF-5, the total price with the tips should fit within the $250 price range.