Boeing and Airbus are starting to get competition from Embraer, Bombardier, Fairchild-Dornier and to some extent BAe/Avro. They're starting to build aircraft that goes above the magic 100-seat mark that used to be reserved for Airbus and Boeing. After the CRJ-900/928JET/ERJ-190 it'll only be a matter of time before the RJ manufacturers build large aircraft, infringing on A320-family/737 sizes.
Now, Airbus and Boeing could just sit back and say "well, our aircraft are better and we are well established with airlines." But complacency could be their downfall, as happened to Boeing throughout the 70s-90s. So, what can Airbus/Boeing do about these larger RJs? They could start their own range of RJs, starting small and building up, but this seems unlikely as neither will really want another large new airplane programme at the moment (although, the Boeing/Illushin deal might being about a succesful RJ).
The advantage of the CRJ-900/928JET/ERJ-190 is that they have comonality with SMALLER aircraft, which will be immensly useful for opeartors who are building up their services and who already have the smaller RJs. In comparison, the A318 is handy for those who have the LARGER A320-family aircraft, and the 717 is a bit of a MD-freak as it doesn't have any comonality with anyother modern aircraft.
So, would Boeing buy out any of the regional manufacturers? Bombardier is probably too big for either of them to takeover. Fairchild-Dornier could be a possible Airbus candidate, especially with its links with Germany. Avro/BAe-whatever could also be another Airbus candidate, with Embraer possibly being a company Boeing could take over.
Whatever happens, Boeing and Airbus are going to have serious competition in the large-RJ catagory.
I hope this rant isn't too ambiguous and i welcome all of your comments!!