For posterity, here's what worked for me:
On Windows Vista; fire up a command window (I have a desktop shortcut "%SystemRoot%\system32\cmd.exe")
cd "C:\Program Files\android-sdk-windows\tools"
mksdcard -l DroidXcard 500M C:\Users\<user>\.android\avd\Android7.avd\sdcard_v 7Run Eclipse and open project. Then:
Run -> Debug Configurations...->Android Application->"Target"<tab>
You may have to resize the "Debug Configurations" window by dragging the lower border down to expose the "Additional Emulator Command Line Options" field.(!) Enter the file path to the sdcard image:
-sdcard "C:\Users\<user>\.android\avd\Android7.avd\sdcard_ v7"Within Eclipse, Run debug (F11), and go the DDMS view (tab labeled "DDMS" in upper right-hand corner of Eclipse window). In that view find the "File Explorer"<tab> which should show an \sdcard directory. *And* you should be able to enter the directory. If you can't you haven't properly created or loaded the mksdcard image. Note: You'll get cryptic "permission"-related messages.
Terminate Eclipse debug session (<cntl>+F2). This leaves the AVD/emulator running and disconnects Eclipse from it.
Again, from a command window in Windows Vista:
cd C:\Program Files\android-sdk-windows\tools
adb -e shell -- fires up a shell on the emulator
cd sdcard -- puts you in the main directory of the sdcard image you created with mksdcard
mkdir <myProjectDir> -- use to create any directory structure you need
exit -- leave the shellRun Eclipse again and open project. Run debug (F11), and go the DDMS view. Using the Eclipse DDMS "File Explorer"<tab>, descend to the directory in /sdcard you want. In the upper right-hand corner of the "File Explorer"<tab> find icons for "Pull a file"... and "Push a file".... Use the latter to move a file from your Windows file system into the emulator's \sdcard file space.
There may be better ways, but for this newbie, it worked.
P.S. Why go to such lengths when you have a real DroidX? Because when you connect your device via USB, Windows auto-mounts the \sdcard directory depriving your device-resident app of access... <grumble>.