Ok, I'll start off by saying that I have limited experience with Macs, and thus won't be recommending any software, but I think I can give an outline as to what you are looking for.
Now, would a translation of your original question be something like:
"I've heard that the 27" iMacs can use their screen as a monitor for an external source (i.e. My MBP), can I work it the other way around?"
I think the simple answer is no. From what I understand, the 27" iMacs are the only ones at the moment that support bi-dierctional Mini-Displayport.
I would imagine that the things that you have been linked to so far work along the lines of either input-direction or streaming solutions.
Input-Direction essentially uses 2 computers, each with their own desktops, and allows your input peripherals to more seamlessly from one to the other (by transmitting the signals over the network). I've used this method for getting a 3-display multi-screen setup before, and it works quite well if you want a few extra screens to hold reference material, or do low-use tasks (MSN, etc), but doesn't actually expand the screen real estate of the iMac.
A streaming solution tells your iMac that it has a second monitor, and streams the data for that across the network to the MBP, which shows it in a very similar way to when you stream a video over the internet. This has the advantage that you can just treat it as an extra screen, but I think there's a bit of a problem with latency, and will depend heavily on your connection (would recommend it over wireless).
Generally, there's quite a few programs on each operating system to do the job, each with it's own set of strengths and niggles. The ideal solution would be a hardware one where the MBP supports displayPort in, but I don't think the current revision do (although I wouldn't be particulary surprised to see it in the next batch of updates for it).
