In detail:

After giving the Turn to the AI, Umber initiated combat against one of my outposts (and its defender). I accepted it to do the battle manually.

Some time after the battle loaded a diplomatic pop-up appeared, (Tarth bragged a bit).I dismissed it with the dialog option available to me.

At this point the battle was locked, nothing could be done in it.

Aftermath:

Stragely, hitting ESC opened up the menu, and the Back to the Game option returned me to the Strategic view, not in the combat. At this point the game was rather un responsive.

Saved the game in hope of 'locking' the issue and giving it to you, but when I loaded it in the same session it crashed.

Repro is highly... unstable, perhaps it can be done with dev tools. In theory it should be something like this:

1. Let the AI initiate a combat

2. Accept it, head to manual battle

3. Make sure that another AI initiates a diplomatic dialog during the battle

4. Dismiss it with the dialog option.

Note: While it  can cause crash, the bug occurs rather early in the chain.

Dropboxes:

http://dl.dropbox.com/u/70329330/FallenEnchantress/20120401/Dialog_In_Combat_Save_Load_Lock/debug.err

http://dl.dropbox.com/u/70329330/FallenEnchantress/20120401/Dialog_In_Combat_Save_Load_Lock/FallenEnchantress0_86-2012-04-01T17-45-23-787.zip

This is the save I've done after observing the issue, but it won't do much on its own.

http://dl.dropbox.com/u/70329330/FallenEnchantress/20120401/Dialog_In_Combat_Save_Load_Lock/test_save_05.EleSav