Microsoft/Danger have now made the necessary fixes to their network to restore the ability to sync your data. This means that your contacts, calendar entries, to-do lists, tasks, etc. will now sync on the network, just like they did prior to this data disruption. However, you must power cycle your device following the steps below in order to begin the synching process.
But they do add the discaimer that things are still “unstable” and that you should back up all your data. Note that a power cycle is going to Menu -> Power Off, letting your Sidekick turn off on its own and then powering it back up. Do NOT do a hard reset as your data will be lost.