boot the mac mini into target disk mode, (google for the shortcut, I think it is cmd-t..)
connect a firewire cable from mac mini to macbook pro. (not sure if thunderbolt is supported as a boot method yet).
boot the macbook pro, holding down alt key
choose the mac mini hd when you get the screen, should be the one with the firewire icon.
voila, your mac mini is using your macbook pro as a display ( well kinda, more like the macbook pro is using the mini as its boot drive.. ;))
Depending on what you are trying to accomplish, if you're using simple apps like mail and browsing, VNC(Virtual Network Sharing) might do the trick without any hardware connections. It uses a network connection to stream your screen to a client.
For setting up your Mini as a server and your Macbook as a client, I recommend using this tutorial:
Mac OS X 10.4 and 10.5 include the server component out of the box so all we need to do is turn it on.
Open your System Preferences from your blue apple menu.
Click the Sharing icon under the 'Internet and Network' category.
Highlight the Apple Remote Desktop component in the list of items.
Click Start to fire up Apple Remote Desktop service.
If you are NOT connecting with JollysFastVNC or ScreenSharing you need the following steps:
Click Access Privileges to open the more advanced options.
Check on VNC viewers may control screen with password and define a password.
You can close the System Preferences. You're done!
Mac OS X 10.4 - Setup as Client
To connect to your new VNC server from a remote machine you will need a VNC Viewer but luckily there are a number of free options out there.
The steps for setup will depend on the viewer you choose follow the documentation closely and you will have no problems creating a connection.
JollysFastVNC is currently the fastest and actively developed VNC client including many features not available in any other client.
Chicken of the VNC is an older client that is confirmed to work with this method simply connect using the IP address of the Server computer. (On the server use Safari or Firefox and goto www.whatismyip.com)