Will Apple's Mac Remote Wipe delete items on external disk?
If Apple's incredibly vague description of "your Mac" and "the disk" also includes what is attached to it, which anyone might assume — then YES. On the other hand, if fluff marketing jargon and disappearing statements might leave some doubts in your mind, I would say NO. The only way to really know is to do what myhd suggests.



Taken from a cached page published by Apple on Jul 15, 2012 21:25:58 GMT:


  Instant wipe
  
  With FileVault 2, instant wipe removes the encryption key from your
  Mac instantaneously, making the data completely inaccessible. Then
  your Mac performs an entire wipe of the data from the disk. External
  drive support
  
  FileVault 2 supports encryption of external USB and FireWire drives.


Source: Apple - OS X Lion - Over 250 new features. Read about all of them.

Visiting the same (live) page now displays:


  FileVault 2 encrypts your data.
  
  With FileVault 2, your data is safe and secure — even if it falls into
  the wrong hands. FileVault 2 encrypts the entire drive on your Mac,
  protecting your data with XTS-AES 128 encryption. Initial encryption
  is fast and unobtrusive. It can also encrypt any removable drive,
  helping you secure Time Machine backups or other external drives with
  ease. Want to start fresh or give your Mac to someone else? FileVault
  2 makes it easy to clean data off your Mac. Instant wipe removes the
  encryption key from your Mac — making the data completely inaccessible
  — then proceeds with a thorough wipe of all data from the disk.
  
  OS X and iCloud can help find your missing Mac.
  
  OS X and iCloud can help keep your Mac safe even when you misplace it.
  Sign in to iCloud.com from another computer or use the Find My iPhone
  app on an iPhone, iPad, or iPod touch to locate your missing Mac on a
  map. If your Mac is offline when you try to find it, you can ask to
  receive an email as soon as it makes a Wi-Fi connection. You can also
  display a message on your Mac screen so whoever has it knows how to
  get it back to you. And until your Mac is back in safe hands, you can
  set a passcode lock remotely, or even initiate a remote wipe to delete
  your personal data and restore your Mac to its factory settings.