Hmm, using IPs wouldn't help either especially if they are using dialup or a DHCP based isp provider. Right now, i use ATT cable and my IP gets refreshed every 5 days.
this may be far fetched, but perhaps you should have a user database. only registered users will be able to cast a vote. this way you can keep track of who voted and who didn't. people can still cheat by registering for multiple accounts, but most won't go through all the trouble just to cast another vote :-)
-sridhar