I have not studied the API in detail, but I am unaware of any direct connection to the database that would allow you to use postgres/postGIS drivers. The API will, however return data from a query in GeoJSON format. This can be used in QGIS. The following steps will provide read only access on a public table:
First, browse to the table view from your CartoDB dashboard and grab your account name and an SQL query. Your account name is in the URL -- it's the bit between
.cartodb.com. Choose or create a query from the menu on the right.
Launch Quantum and open the python console. Enter the following lines, using the info from above. Make sure you keep the quotes.
cartoName = 'your account name'
cartoQuery = 'the selected query'
Then paste the following code. You may have to press [Return] to execute the last line of code.
cartoUrl = 'http://' + cartoName + '.cartodb.com/api/v2/sql?format=GeoJSON&q=' + cartoQuery
response = urllib.urlopen(cartoUrl)
content = response.read()
layer = QgsVectorLayer(content, cartoName, 'ogr')
Or get the Gist if that's easier: https://gist.github.com/geotom/6468534