The PHP problem, as you have probably noticed, has been fixed. Tech support confirmed that because of the PHP 'upgrade', include files with absolute links produce error messages, although the files in question still load. This could cause some CSS site designs to bust (as it did mine in IE).
For the benefit of others who may be affected by this problem, here is the full solution as provided by my web host:
Example of a problem file:
(1) Use relative paths rather than absolute URLs when including files stored locally on your account
...instead of this:
(2) When including remote files located outside of your domain try using @include rather than include
For more information, please see following PHP bug tracker page: http://bugs.php.net/bug.php?id=24053
I also discovered a new MT plugin which should be able to satisfy even the most rabid standards-crazed designer. Checks are made by the W3C Validator in real time, and the status (a customisable message) is reflected on your newly published page. Thanks to its creator, Brandon Fuller. You can find out if this document is compliant by scrolling down to the very bottom of the page.
I also decided to follow one of his suggestions, and let loose Nathaniel Irons' Amputator on my weblog as well, so all evil ampersands (&) will be reconstituted accordingly, never again posing a threat to my kingdom. Bwahahahahar.