Luke T. said:
At which time of its writing, there were half a million slaves in the colonies, some of which were owned by the man who penned those words.
Absolutely correct.

And you hit a hot button with me, so - BANG!
If the words (when penned) were meant to include slaves, then there was apparently a kind of hypocrisy going on. I say apparently, because we're discussing this without historical context. Without trying to determine if GW, et. al., meant that slavery should be abolished or not when they wrote the Declaration, the context of the situation (often overlooked) may give us some reasons for why the intention may not have matched the actions.
The thirteen colonies were fractious, independently-minded groups of people that didn't really get along. They were ultimately unified - not by a dream, or shared belief - but by a percieved common enemy and economic need brought on by the oppression of King George. Even if the intention to free the slaves was there, any effort made to abolish slavery would have dissolved this fragile union almost immediately. No revolution would have been fought. (In fact, things of far lesser significance - like personality conflicts - almost destroyed the alliance anyway.)
Post-revolution, the United States was still a very fragile and vulnerable entity whose primary focus was on simple survival. The economic engine was still greatly dependent on cheap labor (slavery), so subsequent leaders were not about to rock that particular boat.
This doesn't excuse or justify slavery in any way; all it hopefully does is help put into perspective why the possible intentions of the founding fathers didn't match their actions.
Ultimately, it was these specific words in that document which Lincoln used to justify launching a civil war and freeing the slaves later on... So even if the charter wasn't enforced from the beginning, it was eventually.
The price paid for beginning to correct that injustice was very, very high indeed.
But however high the price was, it was both appropriate and necessary. While I'm ashamed that we ever had slaves, I'm very proud that we stepped up to the injustice, acknowledged it, and then literally tore ourselves apart and reinvented ourselves to fix it. What was done doesn't necessarily make things "right" - you can't change the past - but it certainly put us on the right path for the future.
Heck, injustices performed by the USA abound. Just look at the native Americans, for example. But however imperfectly we execute the charter, it's still the basis for the US government, and is almost diametrically opposed to the charter for the skinheads.
(Edited to clarify some sentences.)