Mini DisplayPort and Macbook/iMac?

I'd like to use the iMac as a second screen to my Macbook.  Apple's site hints that this is doable:

Mini DisplayPort output port with support for DVI, VGA, and dual-link DVI (adapters sold separately). 27-inch models also support input from external DisplayPort sources (adapters sold separately).

I know that my Macbook has a Mini DisplayPort out.  The iMac has the same thing but the 27-inch models also support input from external DisplayPort sources.  So, based on the above quote, what I want to do is possible.

Today, while at Microcenter to purchase Windows 7, I decided to stop by their Apple department.  I told the salesman that I had both a Macbook and iMac and that I wanted to use the iMac as a second screen for my Macbook.  He gave me a Mini DisplayPort-to-DVI adapter.  I thought this was fine at the time, but when I got home and had a look at the back of the iMac, I saw that it also had a Mini DisplayPort...the adaptor that the salesman gave me won't work.  I either need a cable that has Mini DisplayPort connections on both ends or I need to buy another Mini DisplayPort-to-DVI adapter (to use with a DVI cable, putting both Mini DisplayPort-to-DVI adapters on each end of the DVI cable).

Now, Apple's Mini DisplayPort-to-DVI adapters are $20 each.  If I had to buy another, I'd be paying $40...and I also don't have a spare DVI cable, so I'd have to buy that also.  That may add up to almost $50!

I saw this a few minutes ago.  Look at the cost! $6.39!  And the customer reviews are nothing but praises on how easy it is to use....and it's just one cable, with no adapter(s).  Someone told me on the Apple forums that I'd have to buy some $150 adapter...ermmm, he's wrong.

So, I'll order that cable either tonight or tomorrow.

I'll also either use that Windows 7 (Professional 64-bit) on my iMac in Parallels or Bootcamp, or just use it on my gaming machine (which is currently running Windows XP SP3).

[EDIT (4/4/2010):

I found out two things:  I need a $150 converter and the $6 cable won't work without the converter, as I've a mini-DVI on the Macbook, not a mini-Displayport.  I'm not going to pay $150 to use the iMac as a second display.  $150 will pay for a new monitor!  I'll use another monitor before I do that.]