BTW if you intend to have this rating to balance the teams, you'd better have a seperate skill figure for each team, otherwise a person who is a 1337 alien gets put on a hummie team and that lowers the actual skill of the team.
EDIT: Obviously a global skill level would soon strike an average between your alien and hummie skill, but that still isn't really useful.
I think a democratic skill level system might work:
Every player starts as level 1.
You can rate any other player up or down,
but they only get pushed up or down proportional to your level.
Lvl 1 players' votes have no direct affect on level, but can be viewed by mods.
All mods have a level 1 higher than normal to start it off.
(This prevents people from flooding users to rate someone up. Even if there is someway to get round "1 UID, 1 vote")
It would give a quick overview of who has actually just started, who is a n00b (Lvl 0 or below), who is uber1337 etc.
You could also add different catagories: Slayer/Builder, Alien/Human
You could also have user requested rankings, for things like Sniper/Feeder/Weapon skills/Alien type skills, and also clan ranks. (Making it easier to tell if someone is actually in a clan, or just flying the banner)
Another very important catogory would be conduct
(Which would start at a reasonable level, so you can tell they difference between a n00b, and a newbie.

)
EDIT: This is mainly me thinking with my keyboard, so feel free to criticise. In fact I'd feel left out if no one flames me
