Using MBP just as a display (kind of..)

ARD, Apple Remote Desktop.

Chicken (was ChickenVNC)
Timbuktu
I'll add more in a moment.

VNC Viewer
Remote Desktop Connection (This is Microsoft, should work, never tried).

 

If the resolution is set the same on the iMac then yes, it runs full screen, if it does not you can control the iMac with ARD, it is not free though, there might be a trail.
On My Powerbook I can control the AppleTV 1 on which I installed OS X(Slow)it is set to my HD Tv 1080 resolution so on my powerbook I have black bars.
I never actually tried to set it different but I am quite sure you can.
Do a google search on ARD or look on Apples site.
There are others which are free, I think Chicken is, timbuktu is expensive($90)
and ARD could be expensive as well, but you'll find it from here I think.
If not just ask.

 

my experience has been with the built-in osx screen sharing options for local networks and teamviewer for remote viewing.

just thought I would add that remote desktop software works by sending compressed imagery of your desktop, so if you are doing graphic design work:

a) i wouldn't 100% trust colour reproduction
b) is it possible the latency between the machines would be just as objectionable as just using the slower machine for something like this? admittedly thats less of a concern if you're on a fast local network, but i sometimes find that certain gestures/shortcuts don't seem to translate over correctly, so it can still feel more "clunky".

 

This point should be BOLD AND 36 point especially with Adobe software if color matching is crucial. Color profiles can vary CONSIDERABLY from display to display and the hidden gotcha in that scenario is that the software will be using the color profile OF THE HOST MACHINE and not the one you are viewing it on.

 

Just to be clear...

Are you guys saying a remote desktop connection is going to be faster than a native 2008 MBP? Using Adobe CS?

I'd stick with the MBP.

 

It depends a lot on what you're doing, but there are certainly tools which would be faster/more responsive running on a really fast machine and only having to ship pixels to another machine.