As seen in this answer, the link:
...has been turned into:
We don't support any form of link except
edit: this has been enabled as a site-specific renderer for android.se. You will have to re-save any affected posts, however -- but all new posts will behave as expected.
Now that the Market has launched as a website explicitly available to both phones and PCs, where the http://market.android.com/.... link will take you to the webpage on a PC, or straight into the relevant page in the Market app on a phone, this probably isn't necessary any more.
See also QR Code generator