Is your ranked MMR separate from your unranked MMR?

There is a separate unranked MMR - option 3. It is copied from your ranked MMR, if you have one, or starts with a mysterious "default value" if you start playing unranked before ranked. As you play, two MMRs diverge.

Source: StarCraft II: Heart of the Swarm Preview -- Ways to Play video

I can understand the confusion.

You do not have an unranked MMR. You only have only have one MMR - a ranked MMR.

Playing unranked games will let you face opponents based on, your one and only, ranked MMR. This will make you able to practice against players at equal skill level.

You can find further information here: http://us.battle.net/sc2/en/game/guide/bnet-overview

edit Team Liquid thread: http://www.teamliquid.net/forum/viewmessage.php?topic_id=382994

In the privious MLG (first HOTS MLG), they interviewed a Blizzard Lead Disgner, who also confirmed this. (sorry for the missing entry from the interview)

TIP: If you want to practice another race, and you do not want to play at the same level as your prime race. You can change server, EU, North America etc.