T-Mobile web site explains how to retrieve your Sidekick contacts

Microsoft and T-Mobile have been working hard at pulling all of the missing Sidekick data out of the black hole it fell into. They can't be asked to do all of the dirty work, can they? Go to the source link for step by step instructions on how to download your contacts and have them restored on your Sidekick or that new phone you replaced it with. Other steps are being worked on so that in the future, you will have the opportunity to retrieve your pictures, calendar info and to-do lists that all disappeared from your Sidekick faster than "cool" at the New Kids on The Block reunion tour.

T-Mobile Sidekick LX 2009 Specifications | Review

source: T-Mobileforums via EngadgetMobile