A post-release video card firmware update issued by Apple has left some Mac Pro users unable to play back HDCP-encrypted, iTunes-purchased content, notes iLounge reader Joshua Murphy. Following an article published recently about HDCP content protection on new high-definition iTunes movie releases, Murphy notes that late 2008 MacBook users aren’t the only ones affected by the lockdown. Equipped with an nVidia GeForce 7300GT card and Samsung…