Actually, the A320's that are out now with the high bypass engines can do coast to coast without any problems. Our first batch of A320's (from Braniff II) had problems doing BWI-PHX on a hot summer day. But our newer 320's and 319's easily did BOS-SFO or LAX-JFK. We have a 319 that does Phoenix to San Jose Costa Rica without any problems, and we also go to Cancun from Phoenix without much problems (even less now that we have ETOPS).
However, the A320-200's in JetBlue configuration still has trouble flying westbound from JFK to the US West Coast on a full load--I believe that JetBlue cannot fly the plane in full 162-passenger configuration on flights like JFK to Oakland, CA or JFK to Long Beach, CA. The 737-800 with the Aviation Partners winglets could easily fly fully-loaded from JFK to Oakland, CA year-round.