In The Book of Discipline of The United Methodist Church (2004), ¶338 addresses “The Itinerant System” …
The itinerant system is the accepted method of The United Methodist Church by which ordained elders are appointed by the bishop to fields of labor. All ordained elders shall accept and abide by these appointments.
This page at UMC.org offers more detail on pastoral appointments. There could very well be some changes as a result of General Conference 2008 (e.g., guaranteed appointments for ordained elders is no more, which means conference leaders will no longer be forced to find places for ineffective pastors/leaders to serve).
Anyway, the overview states …
The primary goal of the appointment system is to match the gifts and graces of a particular pastor to the ministry needs of a particular congregation at a particular time. This itinerant system, where pastors move from one appointment to another, dates back to American frontier days when circuit riding preachers traveled on horseback from town to town. At that time, bishops matched preachers to circuits four times a year. Now bishops typically fix appointments once a year.
Four times a year? Wow. See also History of the Itinerary.
Finally, umc.org has a commentary on the itinerant system as a strategy for the church’s mission by the Revs. Robert Kohler and Mary Ann Moman (staff with the Division of Ordained Ministry at the General Board of Higher Education and Ministry). While it’s a bit dated (2001), it still has some valuable discussion (see also the links at the bottom of the page for more resources).
Hopefully, these resources will provide plenty of background and information about the practice of the UMC’s open itinerant system.