The short answer is that there is a kind of Markdown support for tables in [Markdown Extra](http://michelf.ca/projects/php-markdown/extra/#table) - I'm surprised it hasn't been mentioned in this thread yet (according to <kbd>CTRL-F</kbd> search, at any rate), although we have had Github-flavoured Markdown mentioned.

This has been implemented for a long time, and the "syntax" is pretty well-known. One thinks it would be a simple and effective enhancement for SO/SE. As a bonus, a move to Markdown Extra would bring definition lists, too -- and even [footnotes](http://meta.stackexchange.com/questions/5017/markdown-footnotes) (hurrah!).<sup>1</sup> ;)

<sub><sup>1</sup> SO might not need 'em, but SE most definitely does!</sub>