This is rapidly becoming a hot topic in Records Management with SharePoint, as organisations put increasing amounts of information into their SharePoint environments serious thought needs to be put into how that information should expire and (more importantly) what happens when it expires.
SharePoint has several solutions to these issues which, if configured correctly, can help pave the way to successful document expiration management.
The Expiration Management Policy
Information Management Policies are a new framework introduced in MOSS 2007, and allow specific "policies" to be applied system wide using Content Types, or to individual document libraries and lists.
One of the most popular policies in information management is the Audit Policy (which is vastly superior to version history, as it can track who has viewed, downloaded or deleted a document, as well as who has made changes), but another equally important policy is the Expiration Policy.
The Expiration Policy effectively allows you to specify the retention period for content. This is generally calculated from the created or modified date, although you can specify any formula (i.e. calculated fields) based on any date/time column value.
When a document "expires" you can then select from a number of actions: delete the item, perform another custom action (extensible through development) or start a workflow, and it is the latter that we will focus on here.
As mentioned, you can develop additional "custom actions" for the expiry Information Policy, which you can deploy as a Feature (there is a good article here and also a forum post discussing the options for this).
The Disposition Approval Workflow
The disposition approval workflow is designed specifically for document expiry, and has been designed with a very simple user interface.
When started the workflow creates a task, which is linked to the document. The task presents the user with the options of deleting the item, or keeping it, and the ability to add some comments. Completing the task will perform the appropriate actions on the server.
This basically gives you "out of the box" capability to have items which expire and then get (optionally) deleted upon expiry.
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Now, I put my developer hat on and delved behind the scenes.
The Disposition Approval Workflow consists of a custom Task Content Type which is used for the task, and an InfoPath form which is used for the task edit form (via Form Services).
This gives us 2 options for replacing / extending the Workflow.
1) Create our own custom content type for the tasks. This allows us to attach event handlers and perform any custom operations when the tasks are created / edited / completed.
2) Create a replacement InfoPath form and use that for the task completion, perhaps with additional options (such as "archive" ?).
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Either way, the out of the box options are quite extensive, and the Workflow / Content Type structure gives enough extensibility to provide almost any functionality for document retention.