Google has built one of the most successful email spam filters and they've arguably done a decent job at cracking down on link farms and other black-hat SEO tactics. They'll certainly have more work to do but I don't think they'll have a problem identifying spam or fake "+1's". Historical user behavior and other signals will help them filter out spammers. Additionally, Google will likely emphasize +1's from your friends and my guess is most of us aren't friends with spammers.
Agreed. Twitter is completely inundated with spam and I consider trending topics to generally be among the worst examples of human expression, and yet I love twitter and use it constantly because I only see updates from people I follow.
google's "social" search results don't show up that often for me, but when they do, the little icon and note that says "so-and-so has shared this link" make me look at that link first because a) google has matched it to my query and b) so-and-so liked it enough to share it on twitter (and +1 now? not sure if it's working yet).
This mostly works for me for searches about programming, since that is the community I follow on twitter, but it does a great job of separating wheat from chaff and helping me stumble upon related cool things. Hopefully they don't screw this up and I can get similar benefits for other topics/related communities.
As noted in TFA, as well as on the product page itself (http://www.google.com/+1/button/), +1 only works if you are logged in to your Google account. Spamming it would require controlling a large number of fake accounts, which is not easy.
I see your point, but it's not like there is some sort of +1 on-off switch. Google has some pretty sophisticated and effective means of detecting linking manipulation; I'm sure the same care and set of safeguards would be put in place if +1 was to be used as a ranking factor. Just a casual mention by Matt Cutts that +1 data is used as a part of the algorithm (no matter how small its actual impact) would likely be enough to drive huge adoption.
It would be like a built-in spamming vector for any less-than-legitimate site out there.