I just tested Signal on the PlayBook simulator and it appears to be working. I appended "/iphone/" to the URL to strip off the phone interface so you end up with a URL like http://192.168.1.100:3569/iphone/
If the commands aren't working, can you try bringing the web interface up in a browser on your desktop and see if it runs there? If not you may have been bitten by a rare iTunes bug:
http://www.alloysoft.com/documentation/troubleshooting/itunes-not-responding.html
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Matt Stevens
www.alloysoft.com