Let me answer your questions first before giving you examples...
The acceptance criteria should be defined in the planning phase, this is because it makes no sense to define the acceptance criteria later in the project (when work has started), or before (during project initiation).
The acceptance criteria are defined by the client and the project manager, it is recommended that the project manager asks the client a lot of questions (this is similar to gathering the requirements) in order to outline the acceptance criteria. The project sponsor may be involved in defining the acceptance criteria, but his role is (usually) minimal.
Yes, the "project" acceptance is normally done at the closing phase, but the are several "acceptances" during the project. Usually, there is an acceptance at the end of each major milestone.
Here are some acceptance criteria (for a software project) that I can think of:
- Is the look and feel consistent with what's originally specced?
- Is the database integrity preserved?
- Is the database backed up automatically?
- Is the input validated properly?
Some acceptance criteria for a telecom project:
- If A is calling B, and B is forwarding to C, then make sure that A is charged for the call to B and B is charged for the call to C.
- If A sends a text message to B, and B has is phone turned off (or outside reception), then make sure that the system will retry in an hour, 3 hours, 6 hours, 24 hours, and 3 days before giving up.
- If A calls B and B is on A's friends and family list then make sure that the faf (friend and family) tariff is applied properly.
Some acceptance criteria for a construction project:
- Is the sanitary infrastructure OK?
- Is the material used the same quality as the one listed in the contract?
- Is the internal finishing neat and clean? (when checking for the inside of the apartment, project coordinators use something called a "snag list")
Note that you can be as detailed as you want in defining the acceptance criteria, but the more detailed you are, the more time it will take to get the acceptance (and the more stressful this whole task is for you and the client).