Looks good. Which gui toolkit are you using and do you embed pandoc? Which parts of markdown do you not support and are you planning support for other markup formats like AsciiDoc?
As for Markdown parts – Texts 0.3 supports only blocks of plain text, headers (setext), unordered lists, code and math blocks (pandoc extension) and intervals of bold, italic, code, math and links. Only simple links (<URL>) are supported for now.
The idea is to expand syntax gradually, focusing on simplicity of user interaction. There are too many flavours of Markdown out there and several ways of writing the same structure in Gruber's Markdown is not a good thing, IMHO. There is no real need to decide between _italic_ and italic.
The object model of the text that is implemented is as simple as possible: blocks and spans (without nested spans). It shouldn't be a problem to implement essential parts of AsciiDoc and other plain text markups. When these formats get into the product depends on the interest from users. I'm planning to expose an API to allow implementation of formats as plugins.