I'm in the process on getting this working between InfoPath and WSS 3.0. I noticed a difference where for me the List GUID is url encoded, which looks like this:
List=%7B824F9B5A%2DF795%2D4601%2D89CC%2D5D278870CDC8%7D
I found a handy site at http://meyerweb.com/eric/tools/dencoder/ that will translate this value to the normally formatted GUID. ...