About Linux, as stated before in post #14:
Some distro's do allow other characters and might let you start a user-name with a number, but this could get you into trouble.
Which should have shown you that certain Linux distro's do allow some other characters......
I wasn't talking about my specific distro, but from experience over the years with different Linux distro's and Unix, proprietary systems. If you keep to the [a-z_][a-z0-9_-]* set of characters you will not get into trouble. Do keep in mind that there are more Unix/Linux/MS/Apple/etc flavours out there that you might need to communicate with.
If one works on one specific machine it will not matter that much if one deviates from the "standard" (assuming that that specific distro allows it), if you are going to mail, (un)tar, scp (etc etc) from one Unix/Linux machine to a different machine (using different flavours of OS's) one might run into problems (speaking from experience).
One comment about the AIX set: I wonder what happens when you create a user-name like this: email@example.com
Its e-mail will become firstname.lastname@example.org@some.companies.domain
, which spells trouble.......