Over at Practical Travel Gear there’s a post about noise canceling headphones. I think the best thing you can say about them is that they’re cheap – less than $100 or even $50 for some.
But, if you compare the performance to Etymotic Research, Shure, or other in-ear monitors, you get more noise blocked for a smaller package that sounds better and doesn’t take any batteries with the in-ear jobbies. I’ve tried my ER4’s against the older Bose active headphones, and there was no comparison – the Ety’s blocked more noise. The later active models may be more effective, but they require batteries and are much, much heavier and bulkier to carry.