After finally installing Leopard (10.5) osX on my Powerbook G4 (Thesis writing computer), I noticed a strange behavior with MATLAB. MATLAB could no longer open more than one instance of itself. As well, it could no longer open a window once it had opened once in any login session. Strange behavior indeed.
Well, here’s the fix… it turns out that Leopard uses launchd to set the display. So, the old method of launching MATLAB was to set the display to 0.0, but this will fail after the first instance, hence the bug. What you can do is simply remove this line from the startup script in matlab (located within the startup application contents).
The line that was:
$SHELL -c 'bin/'$ARCH'/setsid bin/matlab -desktop -display :0.0 &'
should now be:
$SHELL -c 'bin/'$ARCH'/setsid bin/matlab -desktop &'
Once I changed that, everything started up fine! 
ps- you will know you have this problem if you look at your console log immediately after launching MATLAB and it says something like:
6/29/10 9:03:34 AM [0x0-0x78078].StartMATLAB[23469] Warning: Unable to open display :0.0, MATLAB is starting without a display.
