As you probably know, you can store documents on e.g. a fileserver instead of your SQL content databases. To make it short, some key features:

• Storing blobs not in your content database, but e.g. in a local folder, or a network share
• With the current version only the SPFarm is supported. Meaning ALL content will be stored externally
• After you have implemented an external storage provider, all new documents will be stored in this place. To move old documents, you could do a Backup/Retore of your content.
• Changing a document will create a new blob and not disposing the old one. You have to do this manually (you can implement garbage collection)!
• With the next SharePoint release you might have to rewrite your storage provider, because it will not remain syntactically consistent
• External Storing of Binary Large Objects (BLOBs) in Windows SharePoint Services

I found an interesting project on CodePlex

• Kaneboy's SharePoint 2007 External Binary Storage Component

Unfortunately it is not ready, yet. When I enabled it, I was unable to upload new documents. But I am sure it will be possible one day :-)