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。