Definitely my biggest current beef with Meteor. It seems technically possible and I understand why they would go with Mongo first, so hopefully its just a matter of time.
https://www.youtube.com/watch?v=EJzulpXZn6g
Here's a follow-up talk he did:
https://www.youtube.com/watch?v=zSI68J9wNJ0
Also, there's prototype-grade support for Redis:
https://github.com/meteor/redis-livedata
And that's just the beginning. There are lots more databases that will follow: https://www.meteor.com/full-stack-db-drivers
The current third party packages with SQL support are very experimental and don't support nearly as much as the native mongo driver.
I assume when Meteor does this or if Arunoda does it would be pretty awesome. Especially with PostgreSQL since it supports native live queries.