An update to my previous post on storing SQL BLOBs in SharePoint. Since MOSS SP1 it seems that you can do this in a supported way by implementing the ISPExternalBinaryProvider COM Interface. More info can be found in the WSS SDK. Limitations around using BLOB store provider (EBS) should be noted. Thanks to Itay for blogging about this!
More info on EBS: http://blogs.msdn.com/husseina/archive/2009/12/26/sharepoint-storage.aspx