We love contributions. This question has come up a few times so I thought I'd clarify how it works. For contributions you need to sign our contribution agreement and fax a copy in before we can take code. Just a step we need to take to keep our IP clean and protect the license. This is pretty simple, and you can get the form here:
Once you do this you'll get a nice little logo on your Forum user name and if you'd like I'll send you some Zimbra wear (hat, shirt, or something) after your first significant contribution.