I have written about this a few times in the past, first of all questioning Google's definition of paid links and more recently with my post on linking payola.
OK I have probably mentioned this a few more times than that, but those articles cover the core points quite nicely.
There Needs To Be A Level Playing Field
When Ebay purchase a site like StumbleUpon and eventually includes a footer link, and possibly some contextual links to product pages that are search engine followable, are they paid links also?
They only paid… what… $75M for 1.5M blogs (and growing) located on subdomains