Apparently Darling got the idea after a visit to LA last year where "some drivers have gone to extraordinary lengths to take advantage of faster journey times on the lanes, including buying blow-up dolls and hiring children to sit beside them."
The main criticism of the idea seems to be that the lanes won't reduce congestion at all, and that it would necessitate the building of parking areas "where people could transfer to shared vehicles" (are you listening Stan Collymore?)
Maybe Alistair Darling would like to consider scrapping the idea of a carpool lane and instead introduce Death Race 2000-style rules to particularly bad sections of the motorway:
Hogging the middle lane - £10 fine
Listening to Heart FM - £20 fine
Hanging your suit up in the back window - £30 fine
Displaying any kind of car sticker - £40 fine
Towing a caravan - Lifetime ban