Apart from the legendary Sosenka apparently. Seriously, if a completely middle-of-the-road rider like Sosenka can beat the hour record, guys like Armstrong, Ullrich, Cancellara etc. could have smashed it if they tried. Clean? Perhaps not, who knows how good they are clean, but the hour record doesn't have any tests they haven't beaten a hunderd times already, if they wanted to, they could have 'won' the record whenever they chose.
I see that this has become a mather of pride to some people, feeling that stating the obvious (nobody cares in the least about the hour record) somehow dimishes Boardman's record. It doesn't. Herman van Springel is still mister Bordeaux-Paris, although no one will ever be in the position to beat him. Similarly, Boardman was a great athlete, take pride in that, but don't be foolish and claim his record couldn't be broken.