The bmibaby brand is simply being used to promote the "tiny" fares which bmi are selling on all regional & mainline flights.
This doesn't mean that bmibaby and our 737s will be operating the routes, it's just like Tango fares by Air Canada. Tango was the well-known low-cost spin-off of AC & their fares became available on all AC flights, as opposed to just a few planes painted in Tango colours. (I did quite like the purple!)
I no longer get shocked when bmi makes a major announcement, because it's just another half-witted idea after another, so I'll wait to see what this one is.
Up there with some of the rumours floating around include MAN going back to all mainline (incl regional) flights, and another about bmi launching a premium economy (like economy plus on United) on shorthaul flights after the uproar from certain diamond club pax at the suspension of business class.