I think in reality this is probably one of those conventions that needs to be broken. Most systems now deal with a lot of sophisticated data and are used to correctly processing text which includes spaces. I was delighted to discover that fogbugz (another plug) will accept your email address, your username or your real name as you have entered it as your username when you log on.
This is simply a convention that is still around from the days of 8 letter file names and probably also 8 letter user names. I would suggest you allow it in your web app and let the world follow you. :)
