From http://www.linode.com/faq.cfm#how-do-i-get-my-fair-share-of-... , they state that you get access to the full CPU of the box (if idle/best case), or access to your fair share of the CPU (if fully loaded/worst case). For the smallest plans this could mean 1/40th the overall CPU if under load. They only put accounts of the same size on each machine.

Linode host boxen have dual quad core processors. Each Linode has access to 4 cores.

For their smallest plan (512MB/$19.95 a month), there are an average of 40 Linodes per box. So, on paper, you are "guaranteed" 1/20 of 4 cores. In reality (according to Linode staff), most host boxes are for all intents and purposes idle most of the time, so if you need to, you can max out those 4 cores.