I was searching the forums earlier today about this issue, then I worked out what the problem was.
It's an issue with how the Blackberry seems to interpret the action attribute of a form tag, when the action has reference to a named anchor.
Check the source code generated by si-contact-form, and you'll notice the action adds #FormID -- whatever that form ID is. This led to the clicking of the submit button simply jumping to the top of the form rather than actually submitting. This does seem isolated to Blackberry browsers.
The (less than ideal) way I have solved this is to remove the # addition from the plugin. Open si-contact-form-display.php and remove that reference from around lines 34 and 42. That should fix it. I say it's not ideal because I guess that updating the plugin in future will restore the old code and reintroduce the issue. Still, it gets it working...