There doesn't appear to be that many people who've actually had their speed doubled. Or to put it another way, it seems that a lot of dates, vitually all of them, seem to have been pushed back from middle of this year to end of 2012/sometime next year.
And with Virgin's less-than-stellar record of customer service (the R36 firmware was promised back in Jan 2012), it wouldn't surprise me at all if the whole Usain Bolt promo (a damn good one too), was just a cynical attempt to prevent customers from leaving whilst signing up new contracts to lock in before they defect to BT Infinity.