Logic Pro 8 Logic won't launch

I click on the Logic icon in the dock. It bounces once and that's it. No different if I go to the applications folder. No different if I try to open by double-clicking a project's song file.

What to do? (I'm hoping the answer isn't re-install). I haven't changed my setup in any way since last evening = no new hardware, no new software.

I had this exact thing happen as well. I don't know why. If I recall correctly all I did was re-install it. You can customize the re-install so that only the Logic application installs. Also be sure that you run all the system updates. I think that's all I did.
It is a permissions issue. I had it happen a couple years back and figured it out. It's not fresh in my mind, but try the following. Open your terminal. Type:

sudo chmod -R 775 [then drag the logic app to here from the finder and it will insert its location as the target]

This sets the permissions to owner read/write, group read/write, all others read only and does so recursively (affects all folder contents). You'll be asked for your password. If this does not resolve the problem, repeat this process on your preferences folders and plist file as well as the Logic folder in the Application Support folder of the Library. Do this for the global and local libraries.

Sometimes changing permissions in the finder doesn't effectively apply permission changes recursively. The terminal is best for ensuring permissions are set correctly and for overriding any strange exceptions.