How to restore Network backup to Network Path in SQL Server 2005?
Microsoft has documented their "Description of support for network database files in SQL Server" at: http://support.microsoft.com/kb/304261 

Trace flag 1807 bypasses the check and allows you to configure SQL Server with network-based database files. However, this is not without risk and you can fairly easily wind up with a corrupt database if the device does not fully support what SQL Server needs to achieve consistency.

If you want to use this approach in a manner that Microsoft will fully support, they have a set of standards describing what they support.  Following is a quote from the link:

Windows Hardware Quality Lab (WHQL) qualified devices
Microsoft Windows servers and networked servers or NAS storage servers that are Windows Hardware Quality Lab (WHQL) certified automatically meet the data write ordering and write-through guarantees required to support a SQL Server storage device. Microsoft supports both application and storage-related issues in these configurations.

Note To be supported by SQL Server, the NAS storage solution should also meet all the requirements that are listed on the following Microsoft website:
http://download.microsoft.com/download/6/E/8/6E882A06-B71B-4642-9EB4-D1EA0D6223C8/SQL%20Server%20IO%20Reliability%20Program%20Requirements%20Document.docx