(2011•东城区模拟)如图,在四棱柱ABCD-A1B1C1D1中,底面ABCD是正方形,侧棱与底面垂直,点O是正方形ABCD对角线的交点,AA1=2AB=4,点E,F分别在CC1和A1A上,且CE=A1F.
(Ⅰ)求证:B1F∥平面BDE;
(Ⅱ)若A1O⊥BE,求CE的长;
(Ⅲ)在(Ⅱ)的条件下,求二面角A1-BE-O的余弦值.