There is an answer over at Superuser. The last post there is what I found to work best, in a slightly different form. The 'checkbox "On"' did not work for me, but the following code did:
# This is only necessary, if AppleScripts are not yet allowed to change checkboxes
tell application "System Events" to set UI elements enabled to true
# Now change the bluetooth status
tell application "System Preferences"
set current pane to pane id "com.apple.preferences.bluetooth"
tell application "System Events"
tell process "System Preferences"
# Enabled is checkbox number 2
if value of checkbox 2 of window "Bluetooth" is 0 then
click checkbox 2 of window "Bluetooth"
You can execute this script using osascript.
Update: I now changed the script to just enable Bluetooth. No toggling anymore.