The only record worth going for is the one with the athlete rules. Boardman's 56 was done on the best human effort rules. His athlete rules' record was 49.441 and that has since been raised to 49.700 by Ondřej Sosenka. However, there has to be a question mark around him considering he is facing a life ban for a second doping offence, although he never tested postive whilst breaking his record.
To be honest, I'm not convinced that Cancellara has it in him. For a start, let's not underestimate how hard the record is now. Boardman was also a phenomenal TTer, so that distance should be considered very, very good.
But, the main difference between Boardman and Cancellara is that Boardman was always very good on the track which gave him a perfect riding style for the hour record. Not only does Cancellara have no real track experience, I don't think his pedalling style is suitable for it. Because of his sheer power, he has a real stomping style on the bike. The road forgive that (and even then, some courses less than others), but I'm not sure that the track would.
Indurain and Rominger had beautiful pedalling styles despite their lack of track experience. That's the only reason I believe that they were quickly able to jump from road to track.
Boonen and Cancellara mentioned at the start of last year that they were interested in riding the pursuit in Beijing. It is believed that both did do tests before quietly dropping the plan. I believe in both cases it was because they realised that they aren't track riders. Maybe with a lot of investment they could, but there is nothing on the track important enough to invest that effort, hour record included.
Graham.