This has to do with Picture Styles, or whatever the equivalent is for your particular camera. When you first import images into Aperture, it will initially display as a preview a JPEG image which is actually embedded within the RAW file by the camera (which is always there, even when setting your camera to record only RAW files, rather than RAW+JPEG -- it's just a small preview JPEG, embedded within the one file). This embedded JPEG is produced by the camera at the time of initial capture, and has the Picture Style (or equivalent; from now on I'll just call this Picture Style, substitute as appropriate) applied to it. Then, as it has time, Aperture will replace its preview with one that it generates by processing the RAW file. As has been said by others, different RAW processing software will process the same file differently -- and this applies equally well to comparing Aperture against the built-in processor on the camera as it does to comparing Aperture to Adobe Camera Raw.
If you change your Picture Style to one that reduces contrast and saturation, you'll find that Aperture makes it more saturated than the initial preview. And if you set it to Monochrome, Aperture will make it be in color again.
The question that I have, that I have yet to dig into to find the answer to (though perhaps I will, now that this question has been asked), is whether Aperture has available to it the information of what the Picture Style settings are, so that it could apply those settings to your image, thus mimicking the camera's processing. If I remember correctly, Canon's utility does this, so I presume it is in fact possible; whether it's easy or not I don't know. What is empirically clear, though, is that this currently is not done. I wish it was - or at least that there was an option for that. Perhaps in some future version there will be.
At any rate, I think you will find that this is what's going on -- that Aperture is processing a file in a particular way, regardless of the Picture Style, but it displays the camera-generated JPEG preview as its initial preview. If you shoot the same scene with 5 different Picture Styles applied, you'll initially see five different looks, and then, as Aperture gets a chance to actually render its own previews, they will all end up looking the same. This is because Aperture ignores this JPEG preview once it has generated its own. The generated-in-camera JPEG is only used to allow for quick access to -some- visual preview of your scene (it's a much less processor-intensive task to render that JPEG on the screen than to render a new preview from the RAW file).
So now it sounds like you'll want to find some settings that match what your Picture Style is, and save those as a preset. Or change your Picture Style to match Aperture better. ;)
As a side-note, it's worth noting that the histogram that your camera shows you is generated from the JPEG preview, so changing your Picture Style will also give you different in-camera histograms.