Good job OP - IsDebuggerPresent is probably as basic as it gets for debugging, so I'll be interested in hearing about the new detection methods Rumble Fighter will begin to employ.
Personally, for an anti-debugger plugin, I normally use
Poison with a few other tools, depending on the packer and method of detection (Poison won't do for EXECryptor or Themida, hah).
Regards,
Vincent