The Google AdSense representative at WebmasterWorld replied to a thread about the concept of spreading out your AdSense accounts on a site by site basis.
The reason people may want to use more than one account, is to reduce the risk of Google closing down one account due to a specific violation on one site, affecting all your other sites. Typically, if you have an AdSense account, you can place ads, using that one AdSense account, on multiple sites. If one of your sites is in violation of Google AdSense terms of service, they may terminate your account. Creating multiple AdSense accounts on a site by site basis reduces this risk.
In response to that, AdSense said they typically try to ask the publishers to make changes to their sites before disabling accounts.
Whenever possible, we will ask publishers violating AdSense policies to make changes to specific sites, or disable ad serving to the sites, before taking action on the entire account.
AdSenseAdvisor then adds that "this is equally true for publishers who own one site or 100 sites, those who earn $0.01/day and those who earn $10K/day." Implying that they treat all size publishers equally.
Forum discussion at WebmasterWorld.