I think Series 4 hasn't been confirmed yet, but given that this series has had ratings nudging 6 million, the BBC would be shooting themselves in the foot not to give it a re-commission!
It was first shown in Autumn 2006, though, with Series 2 not until January 2008, and this one in July 2009, so we may have to wait another 18 months for a 4th series - Winter of 2010/11 maybe.
The destruction of the Hub is no reason to assume it won't come back - that's just part of a BBC process of doing up sets every so often because they get shabby. (Look at all those soap opera fires and explosions - excuse for a re-design.) Neither is the odd character death - look at Spooks, which now has almost totally different cast from series 1.
Rumours, though, of RTD moving across to the USA and taking the concept with him - reinvented for a new and younger team... It won't be the same. Could end up turning into "The OC Fight Aliens."