I have found a simple bug and solution in your plugin. There is one place where you used a short-open PHP tag in the code (line 342), which breaks the plugin on PHP servers that don't allow short-open tags. Not to mention that this is also a no-no in WordPress coding standards. But, since it's only one instance, I'll assume that it was an accident and you already knew that!
Just for reference, here is a link to what it looks like with this bug: http://img510.imageshack.us/img510/8885/sicontactform.jpg
PS: This is the only contact form solution I have found with an option for the user to select where the email is directed. So kudos for that! I am a plugin developer as well and was about to make one myself until I found this. Thanks.