I was having the same problem as lili4n4 describes in initial post.
I normally use Fire Fox 3.0.1 for linux, however the forms were not sending, just sitting there with the spinning arrow image.
So I have tested in Konqueror (KDE browser) and IE6 (for linux). Both these 2 browsers work fine.
I have also had someone test using FF3.0.1 for windoze and that also fails to send form with same symptoms.
Based on these tests this looks to me to be a browser compatibility issue.
You can test it for yourselves on this site:
Site is in spanish, but the form is pretty obvious.
I am using version 188.8.131.52 of the plugin
Hope this helps iron out any problems, thanks for a great plugin !!