http://www.essency.co.uk/awareness-the-headphone-app/

Awareness has become quite famous for this. In fact i was about to post the very same question: we need one for the N900! (it says on the bottom of the page that pc mac android and symbian will be getting their versions soon...)

Now i dont know anyone who has used this app, so if anybody reading this has a <gasp> iphone 4/ipod touch with this app then could they please post thr experience of it?

@shadowjk: i guess one could fiddle with the distance if one had the noisecancelling algorithm to begin with...either way, ive noticed the N900 does have some sort of cancellation already (noted while using realler app to record using 'dictaphone' setting in an echo-ey room with whirring fans, then playing back in headphones....surprisngly good quality...)

It would make music very worthwhile on th N900...