Looks promising but it lagged out real bad after about 30 seconds. You might want to check out your garbage collection technique. 2/5, If the lag was sorted out I'd give it a 4.
I managed to get the little guys across after realising you can add beams to the central piece of land. This is great, really. I'm giving it 4/5 even in it's unfinished state. It will be a definite 5/5 if you finish it off.
wmarsh - I highly encourage you to finish building this game. I love these kind of games and it looks like you've got the main mechanics in place.
Please finish it - if you don't I'm tempted to build the game myself ;-)