Thanks fastair...

The MRJ not coming in to service until 2015, and not to Skywest until 2017... ("It will be the first airliner Japan has designed and produced domestically since the NAMC YS-11 of the 1960s.")

So, Embraer and Canadair have a lengthy head start and more immediate ability to deliver...

If aircraft of this size - with 12 F seats by the way - allow UA to fly you on routes that they otherwise could not profitably do, what is bad about it?

I'll still probably look and see if UA or a competitor offers mainline to get me where I want to go, but if they do not this plane looks like a significant upgrade over the current ERJ/CRJ fleets...