ActiveSupport 2.0.xのto_datetimeが使えね〜
"2008-06-04T18:00:00+09:00".to_datetime
=> Wed, 04 Jun 2008 18:00:00 +0000
"2008-06-04 18:00 +0900".to_datetime
=> Wed, 04 Jun 2008 18:00:00 +0000
cf.
DateTime.parse("2008-06-04T18:00:00+09:00")
=> Wed, 04 Jun 2008 18:00:00 +0900
DateTime.parse("2008-06-04 18:00 +0900")
=> Wed, 04 Jun 2008 18:00:00 +0900
タイムゾーンは無視か。