I'm going to suggest what I do on my website and feel free to use it a food for thought.
So let's say I want to show information about my sponsors and finance and all that jazz. I have a tab on the nav bar that is labelled "Finance" and under that are the subtabs of Current Sponsors, Business Plan and Corporate Sponsorship. The user can click any of the subtabs so go to that page, but the actual box of 'Finance' is a null link. To clarify, a snippet of code is below.
<li><a href="#">Finance</a>
<ul><li><a href="sponsors.html">Current Sponsors</a></li>
<li><a href="business_plan.html">Business Plan</a></li>
<li><a href="corporate.html">Corporate Sponsorship</a></li>
</ul>
</li>
So in the case of your sponsor page my suggestion would be make two subtabs of something like 'Our sponsors' and 'become a sponsor'. That way the user would know that if there were no subtabs then use the main tab but if there are subtabs, use only those.
Just a thought.