i'm pretty sure you can vote for ANY song that was released for that year. at least once they have had a song in the 100 that no one had heard of until triple j played it for novelty value. and if you can't find it on their list, theirs a 'write-in' sort of function. you can even vote for britney spears if she had a song in 2004. the guys on the afternoon show say that if the theme tune from a show called 'media watch' gets in the top 20, one of them will run naked at a big music festival...
one of my favourites for the year was dresden dolls- girl anachronism. dunno if it gets airplay in the US but triple j does. another that probably doesnt get much airtime in the US is cody chessnut. and yet both those acts are american!
edit: i ended up voting. hey, i could do with 90 cd's of the latest artists! this was my 'short list':
*i'm so post modern - bedroom philosopher, the
 chicken payback - bees, the
 *get your hands off my woman - ben folds
* favourite jeans - bertie blackman
 chicken - binder & krieglstein
 10am automatic - black keys, the
 banquet - bloc party
 *yo mama - butterfingers
 city lights - casinos, the
 galvanize - chemical brothers
 get ready - concord dawn
 *a town called sadness - dan kelly & the alpha males
* a to z - darren hanlon
*i'm going to execute your ex-boyfriend - david mccormack & the polaroids
 godhopping - dogs die in hot cars
 girl anachronism - dresden dolls, the
 memory lane - elliott smith
 mass destruction - faithless
 beginning of the ened, the - gentle ben & his sensitive side
 power is on, the - go! team, the
 guns don't kill people, rappers do - goldie lookin chain
 *radio stereo - good buddha
 playing in the distance - grand national
 lotion - greenskeepers
 i'm on you - izabo
 never far away - jack white
 ballad of hank mccain, the - john zorn & mike patton
 school spirit - kanye west
* adrenalin - koolism
 sing song sung - la rocca
* tonights the night - little birdy
 portland oregon - loretta lynn & jack white
* gonna try to be a girl - love outside andromeda
 drum 'n boogaloo - mo'horizons
 letter from belgium - mountain goats, the
 too drunk to f**k - nouvelle vague
 you lot - orbital
 ghetto musick - outkast
 hold me now - polyphonic spree
 shake your blood - probot
 amerika - rammstein
 manilla {remix} - seelenluft
* f***kin awesome - spiderbait
 fit but you know it - streets, the
 whatever happened to corey haim? - thrills, the
 make it rain - tom waits
 staring at the sun - tv on the radio
* summer days - tzu
* no rider - urthboy
 all you people - vanlustbader
 c'mon c'mon - von bondies, the
 troubled mind - wild billy childish & the buff medways
 common people - william shatner
* dimension - wolfmother
 misty rowe - young heart attack
* lachlan - your wedding night
ones with asterisks i know are aussie.
and my final ten were:
favourite jeans - bertie blackman
10am automatic - black keys, the
get ready - concord dawn
girl anachronism - dresden dolls, the
memory lane - elliott smith
mass destruction - faithless
radio stereo - good buddha
ballad of hank mccain, the - john zorn & mike patton
ghetto musick - outkast
common people - william shatner
yeah, yeah i know. william shatner 

 . but it's a cool song, honest!