This is nothing new, and we've had several threads about this in the past. Delta Shuttle is packed during the business week, but on the weekend, loads are quite light, so Delta places some of its Shuttle aircraft to Florida where the loads are higher on weekends. They fly Friday BOS-ATL (maybe LGA-ATL too?), then between ATL and Tampa and Orlando, then back up to Boston. US Airways Shuttle does the same thing, bringing its Airbus 320s from LaGuardia and Boston down to Fort Lauderdale and Miami (specifically due to the influx of cruise ship passengers who generally dis/embark on Sat or Sun),