Respectfully disagree.
Like I said, a very fine impersonation. Impersonating however, is not acting. Not to me anyway. Heath wasn't pretending to be a person who had previously existed, he didn't have the luxury of studying video tapes and copying voices and manerisms. He had to make this wild character out of nothing, with all his extremes and yet make him seem so real and believable.
As um... quirky as this argument is

I agree. I believe it would be far more difficult for a straight man to feign utter, uncontrollable passion towards another man, while also believably acting extremely repressed in 1960s Montana and torn by his own feelings, to get over any and all discomfort one might have during those intense love scenes (though they weren't that long in the film, they would have been shot for days to get them just right...) and I think that kind of bravery and openness as an actor is far more difficult and impressive than doing a funny voice.
To me, though I may be wrong, this comes across as though you've not even seen the film "Capote", and have just decided it's a mediocre performance. I don't necessarily blame you, it's an easy response to make in the face of disappointment. I really wanted Scorcese and Daniel Day-Lewis to win Best Director and Best Actor respectively for "Gangs of New York", so when Roman Polanski and Adrien Brody won instead for "The Pianist" (which I hadn't seen), I resented that, and concocted a bunch of reasons in my head for why they could have won, politics etc, and telling people "The Pianist" wasn't all that great. Then when I actually saw the film, it was great, really blew me away. And I regretted my ill-informed, premature, biased remarks from before. I think you may find yourself in the same situation should you see Philip Seymour Hoffman's performance in "Capote".
It's easy to dismiss Philip Seymour Hoffman as just "putting on a funny voice" in "Capote". But you conveniently neglect to mention that Hoffman, like Ledger, was a straight actor playing a homosexual, and, like Ledger, had romantic scenes with men. And while you may think it's easy to play a real person, it comes with a whole new kind of baggage. Namely, you're battling against people's own memories of the original by giving your own interpretation, and trying to balance putting your own creative stamp on the performance with staying true to the actual person.
Truman Capote is a figure still looked on fondly in New York society as a camp, quirky character, and Hoffman captures that, as seen in the marketing. But he goes beyond that, and beneath the surface to perform a portrait of a man who is actually petty and vindictive, and - as opposed to the image the real Capote created of himself - actually very serious. It's a portrayal that in lesser hands would make Capote an incredibly unsympathetic protagonist - a hateable one, even - but Hoffman draws out the humanity of it, underneath all the other layers, and makes us connect with this man's unique, off-key vision of the world.
I hate this trend on these forums of having to bring down "the other guy" in order to support "your guy". Ledger's performance was Oscar-worthy, yes. But Hoffman's performance was equally deserving of an Oscar. The deciding factor probably ended up being that "Capote" was the latest in a long, long line of great performances by Hoffman, none of which had received Academy Award recognition. It seemed like Ledger was still young, and had a long, great career ahead of him, plenty of time to give him an Oscar down the line. Tragically, this has turned out not to be the case.