"Same-sex couples in Dane County that register as domestic partners with the state beginning Monday won't get all of the same rights..."

"... as their married peers, but they’ll be equal in at least one way — signing up will cost $115, the same as for a marriage license... By signing up, same-sex couples will get a document that formalizes their relationship and can be used in a variety of ways..."

What ways? The more similar domestic partnership is to marriage, the stronger the argument that it violates the Wisconsin constitution:

[Wisconsin Family Action] filed a petition for original action with the Wisconsin Supreme Court, asking the Court to declare the domestic partner registry Gov. Jim Doyle authored in the 2010-11 state budget unconstitutional and to issue a permanent injunction against it....

Here's Art. XIII, sec. 13 of the Wisconsin Constitution:

Only a marriage between one man and one woman shall be valid or recognized as a marriage in this state. A legal status identical or substantially similar to that of marriage for unmarried individuals shall not be valid or recognized in this state.

It's all about "substantially." Proponents of the new law want it to be as substantial as it can be without being substantial within the meaning of the constitution. Similar, but not that similar.

Meanwhile, the state will accept your $115 fee for... almost nothing... or nothing at all if it's is too much not nothing and therefore unconstitutional.