Hi there Chazz. I am not offering public support for Ning 3.0 CSS since it is a feature only accessible by site creators. However I can give you some basic CSS tips that might be of help to you.
Try adding the following to your subtab CSS with that bottom: 100%; value.
position: absolute;
Also, have you tried using the !important tag?
If you need Ning 3.0 specific help I have created a group at our Ning site for network admins. click here to check it out. Membership is granted with a minimum donation of $20/year.