(This only applies to groups, as the actual limit for the sAMAccountName field is 256 characters. Users have a much smaller limit of 20 characters for this field.)
I was originally using CREATE LOGIN with a new AD group (getting the familiar "Windows NT user or group '...' not found. Check the name again."), and traced the problem back to the SUSER_SID() function -- presumably the logic for converting a login name to a SID is centralized, and so affects more than just what I was trying to do.
I tested against many versions (2005-2012) and editions on both x86 and x64 with the same results.
Since it's taken this long for someone to hit this bug (I couldn't find another instance of it being reported), even if it doesn't end up getting fixed, please document the limit in BOL.