About 9 years ago, the (USA) FAA had us switch to new aircraft type
designator codes, which were different then what we previously used for
some aircraft. This, like airspace types, four digit identifiers, and
METAR/TAF weather formats was part of the greatness of International
standardization. So fixed gear PA-28s became P28A (i.e. slower
Cherokees 140-181) and P28B for the faster fixed gear Cherokees (235,
236) etc. See http://www.faa.gov/atpubs/ATC/Appendices/atcapda.html

Recently a speaker from our local Class B Tracon mentioned that they
didn't care what type of Cherokee somebody was flying, they were all
entered as PA-28 anyway.

So does the FAA use those identifiers that they made sure we knew about
and use? Usually when I file a flight plan with a four letter airport
ID, it gets converted to the old 3 letter anyway. Do the FAA computers
and radar systems still accept/want the old identifiers?