今年のクリスマスにリリースが予定されている Ruby-1.9.1 ですが、 Rails側でも対応の動きがあるようですね。

[PATCH] Ruby 1.9 compatibility

Aiming for compatibility with Ruby 1.9. The initial patch is incomplete, though nearly all tests pass.

The only substantial breakage is that 1.9 adds a to_time method that converts Time, Date, and DateTime to Time in the local timezone. We have similar methods in Active Support which convert to time in the UTC timezone. The patch overrides the Ruby 1.9 methods in favor of our own.

期待。