Today i have the same problem with my OS - FC12.
I need to create user: FNAME.LNAME, and when i tried (with and without-m) i received:
useradd: invalid user name
In comparison, in FC11 i haven't such problem.
The workaround i used is to create user manually:
1. create home directory:
# mkdir /home/FNAME.LNAME
2. adding user TEMPLATEUSER:
# useradd TEMPLATEUSER
3. change /etc/passwd:
# cat /etc/passwd | sed -e "s/TEMPLATEUSER/FNAME.LNAME/g" > /etc/passwd
4. change /etc/group:
# cat /etc/group | sed -e "s/TEMPLATEUSER/FNAME.LNAME/g" > /etc/group
5. change /etc/shadow:
# cat /etc/shadow | sed -e "s/TEMPLATEUSER/FNAME.LNAME/g" > /etc/shadow
6. change permition to home directory:
# chown -R FNAME.LNAME:FNAME.LNAME /home/FNAME.LNAME
Please note, that this is workaround, and you could have problems with script that relay that you haven't dots in your username.