There is a lot of absolute drivel written here from self appointed non riding scolds regarding the so called illegality and dangers on motorcycles from I.E.M's.
There is nothing illegal about wearing In Ear Monitors.
I use a set of Jays Q-Jays under a Shoei full face on one of several bikes I own including a 2009 Triumph Tiger 1050 with Remus pipe and two Yamaha FJ1200's with 4-1 pipes.
None of these bikes are as loud as an open piped H.D.
With the ear phones hooked up to a Garmin Zumo 550 and Starcom digital intercom, I can clearly hear traffic, the bike and just about everything around me as well as MP3's, GPS Navigation instructions,mobile phone and even radar detector tones should I wish.
What I do not hear is the constant roar of the wind, that is the dangerous deafening noise.
The attenuation offered by the Q-Jays , indeed any comparable I.E.M is a superb safety adjunct.
Longer rides in particular are not only far more enjoyable with the IEM's,they are far less tiring.
I agree with Giovanni that in slow traffic and on short trips you do not want the music cranked to 11, use good judgement and you will be fine.
Try to get the smallest pair of IEM's you can, having the helmet press the larger ones against your ear can become painful after a couple of hours.
With the small size of the Q-Jays, no problem.