there is a significant post concerning tithing on Scott the Heretic's blog. Scott Klienman wrote the first part and then pasted a reply from a facebook message he recieved from a teacher. you can find it by cruising south on this page under "commune bloggers."
i found it liberating. for the last couple of months i've asked God to help me understand tithing and what the heck it is. this teacher's reply may do the same for you :)