Funny. My local association of electric auto owners just started a similar thread. Here are some of their ideas (note - not sure how many digits/characters you get in your state, buy in Missouri, six is the maximum:
AMPEATR
AMPLE
AMP ME
CHARGE
ELECTRC
GASLESS
KILOWTT
NO GAS
OHMMY
REVOLT
SMOGLSS
SMOKLSS
VOLTAGE
WATTSUP
WEVOLT
ZEROGAS
WTFLUX
PLUGIN
And I submitted this one, which I've seen in photos of a Tesla in California:
LOLOIL