A few more points to add:
No Piedmont planes ended PI because the letters I and O are not permitted, to avoid confusion with the numbers 1 and 0 respectively.
Many carriers choose to end their tail numbers with letters identifying the carrier, but some others like AU and VJ are due to marketing gimmicks, inside jokes, or assorted other reasons. Often times when planes change hands the numbers aren't changed for simplicity's sake. Some ex-Chautauqua SAABs now at Shuttle America still have the NxxxCQ where xxx is the serial number that CHQ used, they haven't yet been reregistered in SZ numbers.
As for the ERJ's in service at U, Trans States' HK is the owner of the company's initials. Chautauqua's SK is for Solitair Kapital, the leasing company that financed the planes. CHQ's newest jets (in DL livery) are registered NxxxRP, for Republic Airways, CHQ's new parent company. Mesa's MJ is for MesaJet, the original name of their ERJ operating subsidiary. There are many more unique cases, these are just the ones off the top of my head..
..CT