Virgin usually have 2 flights going out to orlando on each day, however if the 2 flights dont get enough seats filled they dont fly both flights and therefore cancel one. You will usually be transferred on to the next flight of the day - or if it is a small amount of people it is possible that they can transfer you onto another day.
The other thing is that
Virgin Atlantic operate completely seperate to Virgin Holidays (although its the same brand it is a different company) so it takes a few days for the info to get accross...irritating i know.
Also it says on the virgin atlantic website that the flights that have been cancelled are -
Between London Gatwick and Orlando:
VS027/028 LGW-MCO-LGW - 7, 11, 12, 13, 18, 19, 20, 21, 25, 27 Sept 2005
VS049/050 LGW-MCO-LGW - 4, 10, 17, 24 Sept 2005
Passengers will be rebooked onto the VS015/016 London Gatwick to Orlando services on the same day.
So it probably just means your on an earlier/later flight! Hope this helps!