Just a note, it seems to happen more when MCM is on the my second monitor. 
Could be coincidence though :)

It just completely locked up again (on the primary monitor), this time without 
continuing. The log files stopped 15 minutes ago.

Log tail:

8:51 p.m. :: 
--------------------------------------------------------------------------------
---------
8:51 p.m. :: MCM:  No meta data exists, created a new one!
8:51 p.m. :: MCM:  media details saved using MediaInfo library
8:51 p.m. :: MCM:  Saved the default movie poster
8:51 p.m. :: MCM:  Downloading and saving all available backdrops...
8:51 p.m. :: MCM:  ... backdrop #1
8:51 p.m. :: MCM:  ... backdrop #2
8:51 p.m. :: MCM:  ... backdrop #3

85MB RAM used. Process inactive (using 0%).

Same log file after restart (no exception reported):

8:51 p.m. :: MCM:  Downloading and saving all available backdrops...
8:51 p.m. :: MCM:  ... backdrop #1
8:51 p.m. :: MCM:  ... backdrop #2
8:51 p.m. :: MCM:  ... backdrop #3
9:05 p.m. :: Media Center Master v1.27 (build 27410.450)
9:05 p.m. :: 
9:05 p.m. :: Use the menus to update your preferences and scan folders.  When 
you're done, you can right-click the list of detected videos for specific 
options.  You can also 
drag-and-drop||MCM||URL||MCM||http://en.wikipedia.org/wiki/Drag-and-drop from 
Windows Explorer for direct fetching.
9:05 p.m. :: 
9:05 p.m. :: Example:
9:05 p.m. :: If you have the folder D:\Movies with all of your movies in it, 
then
9:05 p.m. :: add that location to your scan folders and you should see all of 
your
9:05 p.m. :: movie videos listed on the pane to the left.  Now right-click on 
that
9:05 p.m. :: list for meta data fetching and media organization options!

I've seen a similar thing once or twice.
The common one I get is MCM *almost* freezing, taking a minute or two to update 
the screen or respond to a mouse click.
That's with the latest few versions & running on XP Pro with 4GB Ram & a fast 
quad core CPU. Any other program will run fine at the same time, it's only MCM 
that goes into slow motion.
It seems to go back to normal if it completes whatever file processing it's 
doing and gets to a point it's idle.

Is this when using Auto-Scan?

Yes. I have given quite some time to come back in most cases. The log attached 
was about 13 mins (I restarted it as soon as I killed the process).

The freeze in this case is a complete window lockup. No min/max ability, 
clicking on it causes the default windows "ding", no refresh, no right click 
context etc. 

The rig this is running on is:
OS Name Microsoft Windows 7 Ultimate
Version 6.1.7600 Build 7600
System Type x64-based PC
Processor   Intel(R) Core(TM)2 Quad CPU    Q9650  @ 3.00GHz, 2997 Mhz, 4 Core(s), 
4 Logical Processor(s)
Installed Physical Memory (RAM) 8.00 GB

I have had a couple of different scenarios (all when updating my library using 
auto-scan.... it has had a work over recently as I migrated all my content to a 
more correct format):
1) it keeps processing files in the background
2) it locks up completely

Either way I have never got app control back; but in the former all threads 
have not died. 

It is almost as if the GUI thread (per se) has died a horrible death

Let me know if you need any more information.

Just locked up again in a slightly different way
I was adding a TV series
Selected a network share (it is available, I checked)
Entered "Rome" in the tvdb search box
And it locked up.
Killed the process after 90 mins!

This is still happening in the current build.

Hah, having said that as I re-started it a new version is available. So comment 
#11 was regarding the previous version, the one before 32210.723.

Issue 322 has been merged into this issue.

Is this still happening?  I've changed a lot of GUI stuff in the past several 
builds (1.29+).

Have not experienced this for a while, no :)

Fixed in a later 1.28 build.