Shouldn't be too bad. After all, it's a strict superset of the functionality of the old hash. The only confusion will come when someone looks at some code and notices, "Hey, shouldn't this be an ordered hash?" Then they go google "ruby ordered hash" and this blog post pops up and problem solved.
Basically it'd be nice if you could distinguish between instances where iteration order is important and not important. A separate datatype would do that (at the expense of other things, obviously)
If you're worried about it in your app, you could always do
class OHash < Hash ; end
Over time, though, I think this will be a real plus for people learning programming via Ruby. Non-ordered hashes are non-intuitive if you don't know what "hash" means, and think it's just a dictionary.
The big problem is that most people assume a hash isn't ordered -- so when looking at Ruby source, I wonder if this is going to be confusing for awhile?