Yesterday we brought the good news to Hero owners that Sprint was pushing out the Android 2.1 firmware update - but before you press yes on that update, make sure you back up all your data.
Unlike most Android updates, Sprint's 2.1 one actually deletes all of the "current user data," according to the download page's warning. Google will back up all contact and email-related data, but photos, apps and whatever else you're holding on there will be lost. [HTC via Wired]
UPDATE: Obviously if you're a-storing your data on the microSD card, you should be fine - but just be careful to note down app settings and the like, which may need re-saving after the update.