There have even been cases where pro-rata wasn't enough to defend the pilot against claims of commercial operation. The FAA also has a "common purpose" test they can apply, as well as restrictions on "holding out" (advertising yourself to the public).

In other words, as a private pilot, if I posted on this forum and said "I'll take you on a scenic flight and you pay half the cost" that would absolutely be a violation and the FAA could act on it, if they chose to.

Source: http://www.avweb.com/news/avlaw/186346-1.html

Excerpt: "Understanding the FAA's interpretation of compensation leads to the question of whether a pilot can simply volunteer his airplane and his piloting services to transport passengers to where they want to go, as long as the pilot pays no less than his pro-rata share of the flight expenses. ... FAA legal interpretations have been issued declaring that this arrangement may involve impermissible "compensation or hire." "