Replying my own problem.
In order to remove US input source, for the adventurous few (see warning in previous reply), here are the procedures:
1) Make a backup of your preferences, via Time Machine or otherwise.
2) Download a plist editor.
3) Uncheck ALL input sources in System Preferences > Languages & Text > Input sources EXCEPT for 2, the one you want as primary, and the one you cannot remove. You can add them back later, just disable them for now.
3) Open ~/Library/Preferences/ByHost/com.apple.HIToolbox.<some string>.plist *<some string> is a HEX string (hopefully) unique to your machine host using your plist editor.
4) Under "AppleEnabledInputSources", delete the first node... should be something like:
<dict>
<key>InputSourceKind</key>
<string>Keyboard Layout</string>
<key>KeyboardLayout ID</key>
<integer>0</integer>
<key>KeyboardLayout Name</key>
<string>U.S.</string>
</dict>
5) Save, Login and out of your user, and you're done.
Hope this helps!