first of all, thank you for the tremendous work and effort on making Rails more awesome every day.
I’ve brought up this “issue” on Github last Friday.  Steve Klabnik told me to bring the discussion here, as it’s more appropriate.
The thing is, there is some amount of logic scattered around Rails (and its tests) that depends upon error/exception messages, instead of some attributes on the errors/exceptions themselves.
As Brian Shirai pointed out  error/exception messages are for human consumption.
This is a problem for other Ruby implementations such as JRuby and Rubinius, which implement error messages with different wording.
If needed, I’m available to work on this with coaching/mentoring from someone interested on the core team.