SharePoint storing BLOBs in SQL Server carry its overhead (cost, doc size & storage limits…) and when your farm grows you may want to consider using External Blog Storage API that was exposed with WSS 3.0 SP1, in early 2008. Later a project on Codeplex also became visible, for EBS implementation, but doesn’t seem to be accessible now. WSS SDK has been updated with documentation regarding “guidance” on how to implement your own EBS, but I think its bit high-level and leaves a daunting task for .NET developer. Especially if he is unfamiliar with unmanaged code! Apart from that, Kyle Tillman has a awesome 3 series post that goes to a great length describing the steps he took to implement his own, though he shy away from sharing complete code (employer concerns).
For next version of SharePoint Server/Services (2010), MS is certainly including Remote BLOB storage but there is caveat that your existing EBS implementations will likely be incompatible and you’ll have to plan upgrade accordingly. Read here.
Note: Corresponding to SQL Server 2008 there is a samples project on Codeplex for Remote BLOB Storage (RBS) by RBS Team. RBS is different from EBS for SharePoint, but similarly allows external storage of BLOBs for your apps using SQL 2008.
Now, Pav Cherny has published an excellent article in latest June edition of TechNet magazine further detailing EBS implementation and also sharing the code. Of course, the code isn’t recommended for your production use and is untested. Thanks Pav!