It's potentially dangerous for people working on the supply line and it's potentially dangerous for you and your property. The inverters have to synchronise their output wave forms with the supply, bearing in mind 240v is the RMS value, the actual voltage is more like +- 330v I believe, so in the worst case you could have your inverter outputing +330v at the exact moment your supply is restored at it's -330v peak... that would be an awfully loud bang at best.
The only way to do it safely would be to have a proper generator cutover switch that completely isolated your installation from the supply and to have an inverter that is designed for this kind of operation.
And I wouldn't be trying to fake it with a UPS because there's no telling what would happen to the output transistors in the UPS when the inverter suddenly comes on-line and starts throwing power back at them. The diesel generator is less likely to suffer damage, but unless it was specifically designed for such a purpose I wouldn't be trying it out and of course the same applies regarding the cutover switch to ensure there is complete electrical separation between your source of supply and the power distribution network.
In short, don't think about doing it.
