Hello all,
You actually can write to the external SD Card on the Xoom using apps that support the proper Google APIs. For example, you can use "File Manager HD" on the Google Market to copy files from the internal drive to the external Micro SDHC. There are a few other apps that have been updated to support the proper APIs and allow the writing capability.
When you use File Manager HD, drill down from the top level directory to /mnt, there you will find directory paths "external1" for the external Micro SDCard and "sdcard" for the internal storage. Long press on any file or directory gives you a number of options, including copying from or pasting (writing) to either location.
Following the last Android 3.2.x upgrade, applications have been required to use the proper APIs to write to the external SDCard, so SDCard write support will continue to depend on application developer support.
-Doc