Yes, search engines will index any file with any extenstion, its just that they don't store query strings in their databases. This would mean if they found a link to showthread.php?threadid=325 they would store it as showthread.php, which gives an error.

This hack creates an index of all threads, which is called with a query-string-less PHP file.

The threads themselves will be linked from the search engines with the new URL's, but they remain seamlessly identical to the showthread.php versions. It works well

As far as <meta> tags, that could be added via templates, for all thread pages.