I can tell you for a FACT that you'll never see an A318 or A321 in UA colors. :-)
I've been pretty reliable when it comes to United, so take my word for it.
A318 is *too heavy* and it's not cost efficient compared to the 737... the A321 is TOO Much like the 757, but it's economics don't compare. The A321 is also a different engine, different maintenance requirements, and I believe a slightly different cockpit... so it's not quite like the 757/767 cert (crews are cross trained for both)..
At USAirways, an A321 pilot flies *ONLY* the A321, not the A320 or A319.... not like a UA 757 or 767 pilot can go from one plane to the other..
-n