Anyway ... my short answer to whether the flashforwards are really Desmond's flashes is: no.
And this is not because the producers have said that the flashforwards in the finale were something never seen before in Lost - as I indicted in my What Constitutes Reliable Evidence, I never put much stock in what producers say...
So ... to begin with, flashes and flashforwards are really two quite different things.
A flash - such as Desmond gets - is a vision of the future, from or in or by a person in the present. They may or may be 100% accurate and destined to come true, depending upon the predictive powers of the "flasher". In the case of Desmond, in fact, we can't be sure at this point exactly how accurate his flashes are, and whether what they reveal in the future can be changed between now and then.
I should also add that flashes or visions of the future are common in science fiction - such as Paul Muad'dib's visions in Dune.
In contrast, flashforwards are not. And they are not visions of the future, but actual depictions of the real future, which can't be changed (unless we also have time travel, which is a separate, fascinating issue). Flashforwards, in other words, are like flashbacks - both show real events, one in the future, the other in the past.
You could get metaphysically fancy and say that a flashforward is really a God's-eye vision of the future ... but that's just another way of saying the flashforward is not showing us any one character's vision, but the universe's vision - or, its future reality.
It will be fun, however, to see how Desmond's flashes play out in the new flashforwards of Lost...
Lost New Questions: 1. How Far in the Future? ... 2. Who's In the Coffin? ... 3. Who's Waiting for Kate? ... 4. Who Is Naomi's Boss? ... 5. Is Mikhail Immortal? ... 6. What Constitutes Reliable Evidence?
Lost Season 3 Finale ... Flashforwards
Lost: Keys to What's Really Going On
And see also Lost's Back Full Paradoxical Blast: Season 4 Episode 1
The Plot to Save Socrates
"a Da Vinci-esque thriller" - New York Daily News
"Sierra Waters is sexy as hell" - curled up with a good book