I'm trying to do something like this: 
And here's my first photoshoot: 
This was raw converted into jpeg, no post editing at all. Equipment used:
I was not using continous light. The background was done with black cloth.
How can I improve this?
How to improve my product photo shoot to look like Apple's Mac Pro product site?
I think there are two important things to keep in mind:
The main idea with low-key photography is that you want to underexpose the background so much that the camera reads it as black. It doesn't even matter if the background is black or gray or some other color -- if there's no light hitting it, it's gonna look completely black. You've got way too much light on the background -- you need to make the difference in illumination between the subject and the background much greater.
There's no reason to think that Apple's image came out of the camera that way. As long as you've got some kind of clear difference between subject and background, you easily change the background to whatever you want. This took me a few minutes in Gimp:
That's obviously a long way from the quality of Apple's image. Some of the issues can be resolved with more attention to detail during the shoot. For example, it looks like there's some dust on the subject, and while you can remove that in post it'd be easiest to just make sure it's impeccably clean in the first place. Experiment with the lights to accentuate the interesting details, like edges. Make sure you really nail the focus. Other issues are probably easiest to fix with an image editor, like turning the background completely black.
Actually what Apple (likely) uses to produce the results shown on the MacBookPro vid [html5 visualization] on their website isn't a picture at all. Most advertising, for product photographs that need to look absolutely crisp, are rendering a 3d model created of the object; not photographing the product itself. Modellers who do this within Autodesk Maya do this quite efficiently and using render engines such as Arnold or Renderman can produce results that are much better than any photograph can.
As the last answer noted, a little cleanup (like dust removal) goes along way.
But an alternative approach to what others have suggested is to do this, to really get it to look that way: shoot this in passes with a static tripod and vary the diffused spotlight's penumbra angle. Shoot these images RAW. E.G., utilizing a spot light (only), shoot from the left, right, behind and above - all at the same focal length and same camera position. Re-composite them once you clean up each channel individually. You'll see that the spotlight {front-right, for example] would have different light decay than rear-left, but mix the layers in adobe lightroom while adding that coloration on a particular layer will reproduce good detail under low light.
The other photographs you link to of products are rendered (I can tell from my experience), there wasn't a single real camera involved. The woman in the hoodie though was likely done with the aforementioned process. The product photos can be replicated (somewhat) using this process.
Skim through the RAW image article on wikipedia then do some experimentation within any software tool that can properly edit the layers of it and within an hour or two, you'll get the hang of how this type of image editing can produce near exactly the results you are looking for.
jasoncbraatz san francisco, california
In addition to Caleb's point about having less light on the background or, more specifically, increasing the difference between the light on the subject and the light on the background, take a look at the Mac photo's right hand side. Notice the white portion - ie. the reflection. Notice there is a hard line to this (curved around the subject). To acheive this, hold some white cardboard, standing vertically on its edge, facing your subject. In this case it looks as if the flash is above the subject, but it will spill out the sides of your shot, hit the white cardboard, and it's the white cardboard, lit up, that is giving you the shiny white reflection. This is a good technique for showing the shape of wine bottles and other product bottles too.
