Jensen Ackles came third (with 10% of the vote) on MTV.com poll on which actor should cast as Captain America. John Barrowman (Torchwood) and Alexander Skarsgård (True Blood) came first and second, respectively.
Addressing the issue of 'Captain America' during the Vancouver Salute to Supernatural 2009, Ackles said he thinks that attaching him to the role is all a rumour.
Source: MTV.com, Supernatural Wiki.
For more on Jensen Ackles and the 'Captain America' rumours, check Ackles' news section, here,