T-Mobile has issued a warning that the ongoing, dare we say catastrophic failure of the Sidekick servers could result in permanent data loss if Sidekick users in any way reset their devices during the outage:
"Sidekick customers, during this service disruption, please DO NOT remove your battery, reset your Sidekick, or allow it to lose power."
Scary, but also an urgent reminder to everyone who uses Cloud Services, including iPhone owners with MobileMe, Exchange ActiveSync, Google Gmail/Calendar, or any other, similar accounts.
Best practices dictates that data doesn't exist unless it's in at least three locations: local, local backup, and off-site backup. Cloud covers offsite (so does a duplicate drive at another location), but if any of your data is irreplaceable, even important, you need to make sure it's backed up locally.
You can typically save emails, and export calendars and contacts. You can get cheap external hard drives and either manually copy your files over or get easy-to-use automatic backup solutions (some, like Shadow Copy for Windows and Time Machine for Mac are even built into the OS).
Storage is cheap, photos, videos, information, etc. can be priceless. Back it up now. If you need help or advice, check out TiPb's iPhone Forums.