I got this question when teaching a class about architecting Document and Records Management solutions with SharePoint - is it possible to store the documents outside of the SQL database and the metadata within SharePoint?
Yes, this is possible - take a look at the External storage option for Binary Large Objects (BLOBS) in Windows SharePoint Services and Microsoft Office SharePoint Server 2007.
However, there are some reasons to stay away from it - at least for now - recently some more information was added to the SDK - External storing of BLOBs - Operational limits and trade-off analysis - which contains some interesting information:
The external BLOB storage feature in the present release will not remain syntactically consistent with external BLOB storage technology to be released with the next full-version release of Microsoft Office and Windows SharePoint Services. Such compatibility was not a design goal, so you cannot assume that your implementation using the present version will be compatible with future versions of Microsoft Office or Windows SharePoint Services.
There is however at least one vendor which has written it's own implementation using this API - Open Text Storage Services for SharePoint.