It's more like it's not considered impolite to slurp. I think there are different degrees of acceptable slupring noises depending on the setting, company, etc. If your surrounded by a bunch of salarymen at a ramen shop during lunch, you can slurp as loud as you like.
Many Japanese noodle newbies slurp too tightly and create high-pitched sounds that drive dogs insane. The key is loosen your lips and get a nice wet, juicy sound. More than tradition, I think it makes the noodles taste better (and in the case of hot noodles, you're less likely to burn your mouth).
And while I'm on the subject, if you have a seperate dipping bowl of sauce/broth, pick the bowl up off the table with your free hand, dip the noodles in about halfway and hold it slightly under your chin while slurping. Don't be foolish and dunk all of your noodles in the sauce and let them soak.
And watch ''Tampopo,'' for a definitive lesson on the art of eating ramen.