Just discovered that everything on my Tivo box that was older than a few weeks was due to delete "in 24 hours". No idea where that had come from because I've always had it defaulting to "until I delete" and it was about to delete almost everything on the box in one fell swoop, so not to do with the date it's been recorded, so I can only assume it's either some kind of software update or new instruction Virgin have sent through to boxes as part of it's new "channel lineup".
I've sorted it out on my box by going through and manually re-setting everything to "until I delete" but I just wanted to warn other people to double check their boxes before it's too late.
The warning actually states that the programs MAY be deleted within 24 hours, the TiVo calculates ahead to work out if it is going to need to delete things and every now and again it seems to mess up this calculation (it also assumes you're not going to watch and delete anything yourself in the meantime), if you've got plenty of space on the box for your upcoming recordings then you don't need to worry. I've had stuff on my box for months with the warning displayed.
The response above is still valid, it really should state "may" rather than will. The box has no idea of how much you'll watch over the next few days so it's gauging future disk space requirements on what it does know - how much you're going to record.
You might notice it changing its mind once you've watched and deleted more shows but, on the whole, it's nothing to be overly concerned about unless you're really bumping up against a full disk scenario.