The IRS has issued final and temporary regulations under Code Section 385. These provisions, intended to limit earnings stripping, will enhance the IRS' ability to characterize related party ownership arrangements, purportedly established as debt, as equity instead.
One set of rules provides prerequisite requirements that must be met before the IRS will entertain debt characterization. The second set challenges the debt treatment of debt instruments that are not issued for new capital.
The good news is that the new rules will only apply to large or publicly traded entities.
I have done a short summary mind map of the basics of the regulations. You can download it from http://gutterchaves.d.pr/DwHW and when you run the download it should be viewable in your browser.