Yosemite Install or post-install Boot-up Frozen The screen I'm on does not have a time remaining estimate, that was the prior screen. 

This one simply shows the Apple logo and a plain status bar with no percentage or time indicator. 

Perhaps it's just the boot up screen. 

At any rate, it's been stuck exactly at the halfway point for about an hour. 

No clue what the proper course of action here is.

It's a 15 in MacBook Pro, mid 2009, Latest version of Snow Leopard with no history of install related or hardware issues at all.
This appears to be a variation on the boot cache problem with initial upgrade installations of Yosemite.

The difficulty for solving it appears to be knowing precisely when to press and release the  Shift ⇧  key to enter Safe boot mode, which remedies the problem.

The procedure is explained in this Apple KBase article.

Shut down the computer by holding in the startup button until it powers down. Start the computer, and immediately upon hearing the startup chime (or when there’s any sign of life such as the sound of a fan, or hard drive, or lighting of the screen), press and hold  Shift ⇧ . When the gray apple appears, release  Shift ⇧ . After a few moments you will be prompted to log in, and the words “Safe boot” will appear in red letters near the top right corner of the screen.

Be sure to apply the latest OS X update before shutting down or restarting again if your version of OS X is older than 10.10.2. At the time I’m typing this, 10.10.2 is the current version. Having 10.10.2 should prevent the problem from occurring more than once.