Q: What is a slow dropping translucent black screen indicating a need to restart?

Sounds like a kernel panic. These are usually hardware related, often cuased by faulty (or loose) RAM.

What is a Kernel Panic?

http://support.apple.com/kb/TS3742

and  http://support.apple.com/kb/TS1892?viewlocale=en_US

http://developer.apple.com/mac/library/technotes/tn2002/tn2063.html

More details here:

http://www.thexlab.com/faqs/kernelpanics.html

Useful articles on how to avoid and eliminate kernel panics here:

http://reviews.cnet.com/8301-13727_7-10344626-263.html?tag=mfiredir

http://www.macworld.com/article/2027201/how-to-troubleshoot-a-kernel-panic.html# tk.nl_mwhelp

This is called a kernel panic.

It happens if there is some sort of hardware that is inconpatible, failing or has failed.

It usually has to do with the RAM modules either not seated properly in their respective slots, failing or failed RAM, bad RAM or RAM that is not compatible with the Mac.

One way to test is to Safe Boot from the HD, (holding Shift key down at bootup), run Disk Utility in Applications>Utilities, then highlight your drive, click on Repair Permissions, Test for problem in Safe Mode...

PS. Safe boot may stay on the gray radian for a long time, let it go, it's trying to repair the Hard Drive

Reboot, test again.

If it only does it in Regular Boot, then it could be some hardware problem like Video card, (Quartz is turned off in Safe Mode), or Airport, or some USB or Firewire device, or 3rd party add-on, Check System Preferences>Accounts (Users & Groups in later OSX versions)>Login Items window to see if it or something relevant is listed. Or an errant process eating up RAM.

Check the System Preferences>Other Row, for 3rd party Pref Panes.

Also look in these if they exist, some are invisible...

/private/var/run/StartupItems

/Library/StartupItems

/System/Library/StartupItems

/System/Library/LaunchDaemons

/Library/LaunchDaemons