What you were asking your camera to do (high contrast situation on matrix metering) is essentially what fooled it - which is why you have the blown highlights and the ISO of 800. I hate hate hate any automatic or semi-automatic modes simply for that reason. (yes yes yes, I know you can use exposure compensation, but I personally hate it - just a personal preference)
In your menus, you will be able to go in and turn off the Auto ISO feature. That will save you in the future.
But essentially, in matrix metering, your camera was looking at the ENTIRE lighting situation in the frame. Since you had dark dark in the shadows and bright bright highlights in the full-sun whites, there was too much of a contrast for the camera and it had to come to a 'middle ground' in your frame. So it ended up overexposing your shadows your highlights in order to come to that middle ground. The reason it saw ISO 800 is because the dark dark shadows indicated to the camera that it needed a higher ISO - so it compensated.
Someone already said it, but in the future, switching your camera to partial or even spot metering (does the D90 have spot? I don't know the answer?) will assist you in creating a better exposure. In these high contrast situations, you'll want to expose for the brightest part of your picture and then let the shadows fall in dark. It's easier to clip your blacks and bring them back in photoshop than it is to loose all the info in the highlights because that is not recoverable.
(Oh and the price of the camera doesn't matter in these terms... you can have the $8K Nikon and it will do the exact same thing. Sorry to say it is user error - and I say that with total tongue in cheek!

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