Use the [android] shortcode to convert Android Market links into a QR code for non-Android devices and a direct link for Android devices.
A shortcode is a WordPress-specific code that lets you do more by writing less. They can be used in posts and pages (by default) and are marked by square brackets like [this].
This plugin adds the [android] shortcode to WordPress.
Shortcodes are only enabled in posts and pages by default. However, they can be enabled for widgets and templates with a little code. See this post for more info: