http://en.wikipedia.org/wiki/Probowl#Criticism sums it up nicely....
"The player voting has also been subject to significant criticism. It is not uncommon for the players to pick the same players over and over again; former offensive lineman (and SI.com analyst) Ross Tucker has cited politics, incumbency, and compensation for injury in previous years as primary factors in player's choices among themselves, resulting in players such as Charles Woodson, Ruben Brown, and to a lesser extent Jason Peters repeatedly being selected to the game despite having subpar seasons."
1/3 by fans (basicly a who's most popular or most known vote).