It doesn't support Markdown in the sense that your document isn't saved as plain text, but just about every feature of markdown is available: headings, lists, code blocks, inline code, and horizontal rules are all supported. The only ones that aren't are the inline formatting options (bold, italic, underline, links), but these all have the standard Cmd/Ctrl keyboard shortcuts, so it's basically just as good.

The current beta supports a Markdown pseudo-syntax (# for headings, and * for bullet lists) but not an entire Markdown standard as far as I can tell.