Let us pause and define "lead" and "supporting" performances, since the Oscars force us to make the distinction. I believe a lead role is defined by its narrative placement, not its screen time. Narrative placement can be determined simply by asking "Who is this movie about?" The answer or answers are your lead characters, and thus your lead performances.
Example: Holly Hunter and Anna Paquin, who play mother and daughter in The Piano, have equal screen time. Hunter has no lines of dialogue; Paquin does all the talking. They are almost always on screen together. Hunter won best actress, Paquin won best supporting actress (right). That was appropriate. The Piano is the story of Hunter's character, how she lets herself love and be loved despite a painful past, despite having to make a huge sacrifice. The fact that she has a spirited, gregarious daughter is a detail, a texture, a shade, if you will (and I will). Paquin's character is one part of Hunter's character. In other words, Paquin's is a supporting turn despite having the most lines and face time.
Which brings us to Gyllenhaal's silly nomination as best supporting actor. Given this, it would make just as much sense to nominate Heath in this category, and then he would win outright. I think Gyllenhaal himself has a good shot at winning, but that's only because he's in this category.
An actor doesn't have to "file" for a specific category, but they sure do campaign for them, as Gyllenhaal did for supporting. Why do voters go along with it? If I were voting, I'd mark Gyllenhaal for lead or nothing (and it would be nothing). Now the sheer "weight" of his lead performance will probably steal the Oscar away from deserving supporting actors like Matt Dillon. Pity. Or is it?
Sometimes I shock myself with how much I have to say about this horseshit. Now, what say you? Are my definitions fair? Should Gyllenhaal have been denied a nomination altogether before being recognized in a supporting category for a lead performance? What other performances in recent memory have been mis-categorized, and therefore resulted in an unjust win or loss?
Pictured above: One of these things is not like the other. Safely out of competition with co-star Renee Zellweger, Cat-Zeta stole a supporting Oscar with a leading turn from her real supporting actress co-star Queen Latifah in 2003.