What would have to happen in Michigan for the amendment to be repealed? Would the legislature have to act, or can the people petition for it?
Michigan can have constitutional amendments by petition and could repeal their constitutional amendment without a vote of the legislature. However, they can't actually legalize same-sex marriage by referendum; the (heavily gerrymandered and Republican) legislature would need to vote to allow same-sex marriage. (I suppose they could put same-sex marriage in the state constitution, but that seems far-fetched.)
The only states that I would be comfortable putting this on the ballot (that don't already have same sex marriage) are California, Oregon, and Colorado (I would be comfortable with Hawaii, but only the legislature can enact it).
I'd be comfortable here in New Jersey, as well, and also in Nevada and New Mexico (none of which allow referendums, however).*
*Nevada apparently can vote on constitutional amendments without a legislative vote. However, for whatever reason they are attempting to repeal their constitutional amendment at the moment by going through legislative channels (which requires a vote this session (in 2013), a vote next session (in 2015) and then a referendum (in 2016). I'm not sure why they're not just directly initiating a referendum.