From March 31st through May 31st, Sidekick owners who want to leave T-Mobile because of Danger's closing can get the Early Termination Fee waived. Or, sign a new 2 year contract with a qualifying plan and T-Mobile will offer you a 50% discount off certain Samsung handsets. All of this was revealed in a letter from the carrier to a Sidekickowner.
T-Mobile has set up a web site to help Sidekick owners pick a new handset and make the transition to a Danger-free world. The address is sidekick.t-mobile.com/transition and at the site you will find the tools necessary to move all of your Sidekick-stored info to your new phone.
T-Mobile's response to a Sidekick owner mentions the options that the carrier is offering to owners of the device