@skooty: I could not tell if you were running an Intel Mac or not. If you are running an Intel Mac then there may be something else going on that I have no clue about... but this should just be working for you. But, anyway...
You might wish to run the standard maintenance operations using the Disk Utility first (Repair Disk and then Repair Permissions). Then go into the System Preferences and look at the Software Update section to see if there are any updates that apply to your system, install any that are critical (OS or security updates). Then create a new account, making sure that account is allowed to administer the machine. Log out and login to that new account (if you already have an admin account then just be sure you are logged in as the admin). Then mount the .dmg file from the download and recopy it onto your desktop and try running it from there.
Come to think of it, I have seen this sort of behavior in other applications when the font caches on the machine get corrupted. I am not sure if this would apply to Planeshift, but it could since there are fonts being used by the program. But that can cause strange crash behavior in applications like what is being described by Skooty. Applications (free) that I have used for clearing the font cache files are Onyx and Tweak Freak. It can be done manually from the terminal window, but please check for instructions on that from the Apple support site.
Note: When applications crash under OS X the crash reporter logs the conditions. Either you will get a window that has a Report button on it, which will display the crash. Or you can go to /Library/Logs/CrashReporter/ and see if there exists a file called something like psclient.crash.log. Important: This file can be VERY large, but in my experience it is the header of the file plus the data lines between "Thread 0 Crashed" and "Thread 1 Crashed" that counts. If you try everything and are still getting the crash, look at this report and paste the valid part of it that I just mentioned up here so we can see it. Please do not post the entire crash file, as it can be very large, as I said above. Cha0s or dfryer please post if I am incorrect about the data in that report file, for it seems that I have never gotten anything useful from beyond the Thread 0 Crashed section of the report.
Hopefully that made some kind of sense. But, urg, what a glut of tripe, sorry for the mongo posting, there is some small chance it might help someone.