To add onto doobierw's post.

Recent negotiations increased the number of CRJ's than can have 50 seats. I believe that the remaining CRJ's on order will all be delivered with 50 seats and some of the 44 seaters will be converted to 50. I don't know the exact number though.

Hub over-flying (routes out of IND, MKE, IAD-LAN, etc) are to be flown by aircraft with no more than 44 seats, hence the 44 seaters are placed on these routes.

However, NW recently announced GRR-DCA with the Arvo. The Arvo could hold 85 passengers in single class, but again the pilot scope clause capped these at 69 seats, as anything with 70 or more seats must be flown my mainline. Thus the 2-class, 69 seat arrangement, that was limited to the 36 (now 35) Arvo's operated by Mesaba. The Arvo gets around the hub over-flying clause due to its weight as it is not classified as a regional jet under the pilot scope clause. Therefore, the loophole, allows the Arvo to be used on point-to-point routes, its just that NW has not decided to place it on such routes before.

Stay tuned, with negotiations for 70 seaters underway, this all will likely change more in the future.