As it is very likely that spammers will try to use the +1 button, Google will perform tests before the +1 button has a major influence on the search results.
The +1 button will be shown in both natural and paid for search results so will it effect PPC and will it be easy pickings for SEO spammers? On PPC Google have claimed it will not effect quality scores but have also mentioned that it does increase click through rates. Surely click through rates in turn effect your quality score and therefore +1 will impact quality scores.
If you click twice it will decrease your vote by -1. Spammer cannot much use it as it require separate Google account. Spammer if increase the counter to 100 or 150 I think it will not have major affect on ranking. If they raise counter to 1K it may have some positive signals