The information has been confirmed by Tom Hanks himself on Twitter :

RadioMan delivers msg from T.Fey. Result? I'm on 30 Rock! Thanks, RMan Hanx

This spring, Tom Hanks will guest star in an episode of the NBC comedy series 30 Rock which stars Tina Fey and Alec Baldwin. We don't know yet which episode, when it will air and whether Tom Hanks will play himself of a different character.
30 Rock takes place behind the scenes of a fictional live sketch comedy series depicted as airing on NBC.

This appearance will mark Tom Hanks's return to Network Television, almost thrity years after guest appearance on the television series Family Ties. Tom Hanks has already guest starred as himself in his career, on The Naked Truth (video) in 1995 and Life with Bonnie (video) in 2002.

Guest Stars on 30 Rock have previously included Susan Sarandon, Matt Damon, James Franco and Chloe Moretz.