Boston requires that bikes be registered but the city scrapped its internal registration program in favor of the NBR, but you can't actually get NBR forms from the city. Yet another way Boston demonstrates how it truly values its cycling population.

Fortunately the state law covering municipal registration says the fine for violation is $1 and will not affect your civil rights or liability. So it's not street legal but if you get into an accident with an unregistered bike, the worst that happens is that you may end up paying a $1 fine (though I've never heard of it happening).