If you have a T-Mobile Sidekick 4G, you should receive an over-the-air update notification for a new ROM available. Before you get too excited, Android 2.2 Froyo will still be the platform operating your phone as it’s not a platform update but a maintenance one.
That being the case, your new version after the update will be T839UVKG2. It’s being pushed out in waves since July 21 and you should receive it soon on your phone. Not much info is being disclosed by T-Mobile except for the fact that the update includes “various bug fixes and enhancements based on customer feedback”. If you want to read more about the update from those who already updated, check out the source link and browser through the initial feedback.