Just my 2ps worth but if i see anything with "win an ipad" i mark it as spam and run like the wind cos there are SO many companies (especially on Twitter) that claim to be able to offer you an ipad, and a LOT are scams.
I do think some sort of competition to rally up newbies would be ace, but why not make a monthly design competition or something like that? Granted, retweeting something is much easier!
Maybe for the re-tweet prize drawer have smaller prizes that can be given away more often - pantone mugs, posters, free copy of OSX Lion etc.