Found this on the RO board.  Sounds like it is safer to use a passcode to lock your phones.  I imagine this will be appealed and also tried in other parts of the country but who knows how it will turn out in the long run.

… if a business or government agency uses security mechanisms like retina scan or hand print or voice print, LE can waltz in without a subpoena. Riiight.

No, this will not stand.

This isn't a new issue.  I don't have the cites in front of me, but this has been the case for years.  They can use your fingerprint to open the phone.  However, they can't compel you to reveal your passcode.  
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I use the fingerprint thing for convenience.  But I know if I power off my phone, it won't open on power up with the finger print, you have to enter the passcode.

But a lot can happen between now and whenever this is finally resolved.  I'd stick to using passcodes.

There have been different laws and different court ruling in different places although to the best of my knowledge you are right that they can in Minnesota.  

However, I think this is the first time a federal appeals court has ruled on the matter.  And I think it will be appealed to the full circuit court (this was a ruling from just a 3 judge panel).  The ninth circuit doesn't cover Minnesota so I don't think the decision there will affect Minnesota either way but it's easy to see this ending up before SCOTUS one day.

I think SCOTUS will say you have a right to remain silent (not testify against yourself, i.e. not reveal a passcode) but that you don't have a right to remain invisible (finger prints, face recognition, eyeball scan, etc.)