Ok, so here's my proposal for an attribution guideline when modifying themes:
1%-10% -- Still the same theme. Changes are considered customizations or "flavors" of the original. Copyright and original license must be kept.
11%-50% -- Theme "inspired by" original. Reference to copyright, etc. must note original author. New elements (images, templates, code) not in original fall under current author's copyright (naturally). Original license must be kept.
51%-%90 -- This is a new theme. Original is seen as a "base." Work is considered derivative, but not "appropriated". Reference to original theme/author is suggested. Original license must be kept.
91%-99% -- If I can't recognize any of my theme in yours, I'm not sure how I'd enforce issues of copyright/license/etc. For this, we'll have to rely on trust (I know, but sometimes you have no choice).
100% -- What's this doing here?
All % values are subjective--there's no way around that. However, we can certainly hash out general areas of modification that fall under each. For example, swapping colors is an obvious 1%. Changing a fixed-width template to fluid is probably starting to step into the 11% category.
[EDIT: Made slight changes after posting in regards to licensing. This proposal is also dependent on themes which offer a license allowing for modification and redistribution. Most, but not all, will.]