From the apache log:

[Thu Nov 01 19:49:50 2012] [error] [client <snip>] PHP Fatal error:  Call to undefined method User::isAllowedAny() in /usr/share/mediawiki/includes/specials/SpecialImport.php on line 59, referer: http://<snip>/index.php/Special:SpecialPages

Looks like there is some awareness of this issue:

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=668601

Here's a fix for everyone (credit due where credit due) constructed from information in the above link:

1) Download http://dumps.wikimedia.org/mediawiki/1.15/mediawiki-1.15.5.tar.gz

2) Unpackage the file \mediawiki-1.15.5\includes\specials\SpecialImport.php from the archive indicated in step 1

3) SFTP (I used FireFTP) to your turnkey-mediawiki-12.0-squeeze-x86 server

4) Navigate to /usr/share/mediawiki/includes/specials

5) Rename SpecialImport.php to SpecialImport.php.original

6) Upload the SpecialImport.php from step 2 to usr/share/mediawiki/includes/specials

7) Reboot your turnkey-mediawiki-12.0-squeeze-x86 server.  (This may not be necessary, but I did it to be rigorous)

8) Use Special:Import to your heart's content.

Because I only had to use Special:Import once, I thereafter renamed SpecialImport.php
 to SpecialImport.php.1-15-5 and SpecialImport.php.original to SpecialImport.php