Good for BA. I think this is a great move and one which could do very well for them.
I strongly suspect it has been brought about by bmi's announcement on LHR-JER. BA found that they could sandwich an extra LGW-JER-LGW roundtrip onto the 737 fleet this summer and had planned to increase LGW/JER. However, with the bmi announcement, I guess this has left them feeling less bullish about growth on Jersey and so they needed somewhere of a similar rotation length to use the aircraft.
A daily midday service from LGW to NQY ought to work OK, and if you look at passenger volumes since Ryanair reduced, you can see why. August saw LON-NQY passenger numbers almost halve - from 23,708 in 2005 to 12,036 in 2006. That's 11,672 fewer passengers due to the reduction in STN services. A daily 737-400 generates 9,114 seats per month and so the capacity influx from BA's arrival is far less than the number of passengers lost from the route. In commercial terms, it ought to be a no-brainer - particularly when you can't use an aircraft that you're already paying for anywhere else, i.e. it's marginal use of the aircraft.
For Cornwall, it's great news. It means the airport is less beholden to Ryanair and its tactics - and I see there was another press release on Ryanair's website about it only a day or two ago. I suspect that other posters' suspicions about Ryanair might be right, but let's wait and see.
The only implication is that I guess Air Southwest might move their handling contract at LGW away from BA! Still, given BA's approach to third-party handling at LGW over the last few years, they might see that as a bonus rather than a loss.
Overall, for once, it is nice to see positive moves being taken. Congratulations to whoever had the balls to push this one through Waterside.