Michigan specific information
Michigan does have a section of law regarding traffic control areas. This mostly deals with crosswalks at signals, but portions of it refer to crosswalks in general. See state statute 257.612 which deals with disabled individuals crossing at any crosswalk, not just signal locations.
There is also a definition of school crossing which is very broad. It simply requires that it was designated by regulation to be a school crossing. Children have the right away and vehicles must stop.
Finally, the definition of a crosswalk in Michigan is setup in 257.10(b) which states that the crosswalk is any designated walkway for pedestrian crossing.
Your mileage may vary with regards to getting a ticket for entering a crosswalk, but I would not take the risk. Outside of putting others in danger, there is a very real chance that these statutes apply to you as the driver even if there is not a specific law that says so much in one sentence.
If you run into the unfortunate circumstance that a person is injured or killed by your vehicle, even if the crosswalk is in the middle of the block, the prosecutor will likely use the above statutes to press charges.
The best thing you can do is to stop for pedestrians in such situations. In Michigan, it may be a legal grey area, but in reality, it is the right thing to do.
Original Answer (the question used to ask for a general driving rule in the United States)
As a general rule, states give pedestrians the right of way when in any marked crosswalk or while crossing at any intersection (marked or unmarked).
Many states, including my home state of Minnesota, set up sting operations where the police will secretly watch a corner and hire a pedestrian to walk out into the crosswalk. If a motorist does not stop, they will be issued a ticket.
Failing to stop for a walker where "zebra stripes" are present, even in the case that the stripes are in the middle of a roadway (not at a corner), is usually grounds for a fine.
In most jurisdictions, one must assume that failing to stop for a pedestrian is a ticketable offence.
Here are a few example laws for your reference. It would be too much to list every state and local jurisdiction.