Daniel Radcliffe and Helena Bonham Carter, from the Harry Potter movies, as well as other well known actors will appear (some via video) on a special broadcast of the show titled American Idol Gives Back.
Appearing with Dan and Helena will be Keira Knightley, Hugh Grant, Forest Whitaker, Pink, Gwen Stefani, Bono and Rowan Atkinson, to name a few.
According to the web site:
The show will benefit CPEF is a charitable U.S. organization established to raise money and awareness to help combat extreme poverty in the US and throughout the world, particularly in Africa.
Tickets will go on sale at 10am on Tuesday, April 10th, internet only, log-on to ticketmaster.com. By the way, proceeds from the ticket sales will go to the charity.
The show will air on Tuesday, April 24th and Wednesday, April 25th (please check your local listings). At this time we don't know what dates Dan will be on the show.
Thanks to Belle for the tip.
UPDATE: Thanks to DR.co.uk for letting us know that Dan's appearance will in fact be via video, as previously pointed out and the segement with Dan will not be "exclusive" to American Idol. We will update with more when we can.