You can put your documents in SP - cool, but, when its time to reorganize folders structure what I should do with old links?
Is there a way how to identify document instead of using URL where path to the document is used? I need something like permanent link: http://mysharepoint/doc-123, where 123 is the document identifier.
I need URL to the document which doesn't depend on folders where the document is published. Are there solutions?
You can use an URL Shortener solution and combine it with a SharePoint (Designer) Workflow or Event Receiver.
I have worked on a product that can be used in combination with SharePoint and can be used via web services, workflows as well as the user interface.
As your need is quite specific you may need to do a little bit of extra coding. If you don't have access to Visual Studio or prefer to use SharePoint Designer workflows for everything than have a look at the Workflow Power Pack.
SP2010 will offer a unique per site collection identifier for each document that you can use with a built-in redirector service.
You can create database table that contains the mapping to the files (you'll need to update there when the file changes) the table should contain url wanted to route | url destination then you could implement an HttpModule that reads that url and then redirects to the one in the table, however that will cost you overall performance so you could create a pattern and only do that when it matches the pattern.
In SharePoint 2007 you could create a redirecting page that takes the unique id of a document as query parameter. Use this id to get the document SPContext.Current.Web.GetFile(id) and redirect the response with the file url.