This works well if you setup an AppleScript or Platypus app to turn it on and off. More info on Platypus here http://sveinbjorn.vefsyn.is/platypus .

to make a Platypus app I used TextEdit to make two one-line scripts. One to turn the blob on, and one to turn it off. Put the following line in TextEdit:

defaults write com.apple.dock wvous-floater -bool true; killall Dock

(change true to false for the script to turn it off.) From TextEdit's Format menu choose Make Plain Text. Save the script, then drag-and-drop it into Platypus and click 'Create'. Do the same for the script to turn it off, then put your Platypus apps in your Utilities folder, or where ever you may want to keep them.

Likewise, in AppleScript, this script will run the command in a terminal (if you don't want to mess with Platypus):

tell application "Terminal"
activate
do script with command "defaults write com.apple.dock wvous-floater -bool true; killall Dock"
end tell

It still works beautifully with Tiger