Ooops... try again :-)
using:
unzip -Zl arduino-1.03-osx-digispark-2013-01-08.zip
shows lines like:
-rw-a-- 6.3 fat 51744 bx 11706 defN 10-Dec-12 04:10 Mac/Digispark Ready - Arduino 1.03/Arduino.app/Contents/MacOS/JavaApplicationStub
Which looks to me like it was created from a FAT filesystem which has very limited permissions - in particular doesn't have/need execute permissions. Compare this to the same file in the teensy3 zip file:
-rwxr-xr-x 3.0 unx 51776 bx 11797 defX 12-Jan-13 22:34 Arduino.app/Contents/MacOS/JavaApplicationStub
here the filesystem is 'unx' and this has correct file permissions bits (ie -rwxr-xr-x).
Since the permissions are missing from the Digispark zip file I suspect that means either it was created from a FAT filesystem or that zip stripped the permission - I think the latter is unlikely.
The real fix for this is to repackage it with the correct permissions, however in the interim, given that we don't know for sure which files should be executable but that an execute bit being set where it isn't needed shouldn't break anything, one really, really ugly fix would be to open a terminal, cd into the digispark's Arduino.app directory, then run something like:
find . -type f -exec sudo chmod +x {} \;
which will recursively set the execute bit on every file - but take care.....