Yeah sure, pretty easy really.
Just use the selection brush to highlight the area you want to keep in colour (Make sure you zoom in etc to ensure you have selected all the bits you want and removed all the bits you don't)
Once selection is complete select Inverse selection, this will then select everything outside of your selection.
Then choose convert to black and white
Its that simple really,, you can then add vinegettes and other effects until your please with your end result.
OK - thanks - I'll have a play, but my experience with the selection tools is not brilliant... maybe you are just more accurate/patient than me. I had rather hoped there was some clever selection by colour process. I guess I need to be more patient!