The Meta plugin comes with support for Twitter Cards since version 0.2.0. But, as I have noticed, adding support is not enough for them to work… Twitter won’t display your cards until their usage for you domain will be approved by Twitter staff.
Therefore, right after putting the plugin on my website I sent the request to Twitter for approving my domain (using this page). Usually I deploy the plugin to the site in some time before I release it (to test, how it works in the real environment), so I hoped to get approved before the release. But Twitter guys appeared to be slow in this – I released the fixed 0.2.1 and the domain was not approved yet. Eventually, I stopped expecting the approvement to be done… But today, finally, I got an email notifying, that my domain had been approved!