Often enough that the code has provisions against it. Usually a big red flag is when a submission is only minutes old, is of low quality and has 3 or 4 votes on it.
I think the main reason for this is that if you manage to get a submission on the front page in the first couple of minutes that it exists momentum will take over (the homepage is a feedback loop, once there more people see your submission, so it will get voted up and stay longer on the homepage).
The detection of 'voter rings' (groups of people that vote each other up consistently) is not a simple problem.
I think that to stop a site such as HN from being gamed completely is very complicated, related to the the click-fraud combating efforts at projects like google adsense.
All things considered I'm actually surprised how little it seems to happen here (compared to other sites like HN), one of the main reasons is that it is (on a relative scale) still fairly small.
Making who votes for what public would make it possible to spot such voter rings much quicker.
As for the 'NEW' page being the place to look, I almost exclusively use the 'NEW' page, the 'News' page maybe once or twice per day. A lot of good stuff passes by the homepage simply because it does not attract enough votes.
Another problem is the sometimes sluggish reaction of the moderators to these tricks, (see the link right below you for a nice example), any clicks from HN to a spammed site will increase the chance of repeat actions.
A 'barrier to entry' (say 25 points of karma before you can post your first article) would be another good thing to do.
