Nope, open to anyone.  From their Website
(http://dor.sd.gov/Motor_Vehicles/Procedures_Manual/SectionB.aspx).  Looks
like it will cost about $65/year for a 4000 lb vehicle:

Non-Resident Title and Registration

South Dakota title and registration statutes do not expressly prohibit an
out-of-state applicant from titling and registering in South Dakota.
An applicant must provide the county where the vehicle will be kept and the
applicant's residence post office address (actual address) on the
application for title and registration. If an applicant actually resides
out-of-state, the applicant's actual address must be used on the application
(SDCL 32-3-18 and SDCL 32-5-3).
Residence post office is the location at which a person actually resides.
For applicants that are truly nomads (no abodes in this or any other state),
an affidavit attesting that the applicant(s) has no residence post office
(actual residence) in this or any other state is required before a mailing
forwarding address, rather than a residence post office address, can be used
on the application.
An applicant must provide either a South Dakota driver's license number
(SDDL) or Social Security number (SSN). Applicants who list a SSN and do not
have a SDDL number must provide a photo copy of a picture ID (e.g.
out-of-state driver's license, tribal photo identification) for each
applicant listed on the application. The photo copy of the ID must be
submitted along with the application (SDCL 32-3-18 and SDCL 32-5-3).
Additional information may be requested if needed.
Providing false information on the application for title and registration is
a felony and anyone doing so is subject to further action by the department.

Stuart

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Donna Skarloken
Sent: Monday, June 02, 2014 12:40 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: [Adventurewagen] Need to sell my van : ( Another victim of bad
diagnostic skills! This really bothers me

Yes I would love to know - I would think you need to have a residence there.
My husband has many relatives in SD.

I am bracing for the Cal smog test this week.

Donna 87 syncro