An intermediate (below the state's Supreme Court) appellate court in Michigan has held that the state's marriage amendment bans public employers from offering same-sex domestic partner benefits.
The language of the two amendments is close , but not identical. Michigan's amendment provides that no other relationship other than the union of one man and one woman shall be recognized as "a marriage or its legal equivalent by the state for any purpose" while ours says that no "legal status that is identical or substantially similar to marriage shall be valid or recognized." While Michigan's amendment seems concerned with providing a relationship with any of the incidents of marriage, ours is arguably concerned with providing a relationship with so many of the incidents of marriage that it becomes a marriage equivalent.
A court may conclude that our amendment prohibits only the creation of a status that has most of the legal incidents of marriage, while Michigan's focus is on whether a relationship is treated as a marriage for any purpose. I can see a court concluding that just giving someone employee benefits does not confer most of the benefits of marriage (and is, therefore, ok in Wisconsin), but providing benefits is treating a relationship as a marriage if those benefits are something that are otherwise available only to married couples (and, therefore, not ok in Michigan).
More importantly, the Wisconsin courts (more so than in Michigan) give a lot of weight to the legislative and electoral history of a constitutional amendment. It will look closely at what the legislature said when it passed it and what claims were made about it in the course of the campaign. (It will also look at relevant legislation passed shortly after its passage.) That history is, of course, unique to Wisconsin.
Of course, even under the Michigan decision (which will certainly be reviewed by that state's high court), it would probably be possible to confer benefits if eligibility weren't defined in a way that consciously tracks marriage.