I have a client who wants to share one custom list with multiple users - the problem is each user must be restricted to only be able to view what is assigned to them.  I can obviously create a view that will do this, but I can't lock the user out from being able to switch views that I am aware of.  In advanced permissions for lists you can change the settings so that the user can only view and edit their items - but Microsoft was a bit too literal with this definition, your items is an item you created, not an item that someone else created and assigned to you - that is a common scenario for this client.  The only other way I thought of being able to do this would be to use a subsite with independant permissions for each user - but is there a way to share data from one list between a main site and subsites and restrict what data is shared to which site?  I would think this is a common problem, but thus far have not been able to find any information on how or if it is even possible.  Any input would be greatly appreciated.  Its a Windows 2003 Standard Server with SPS V3.0, database server is Windows SBS Premium w/SQL 2005.

Why not create a subsite with dataviews that only show the view you want them to see (i.e. no view nav). While they may be able to get to the other site just hide it or make it hard for them to navigate to on their own.  

Have you seen this: How to restrict public access to sharepoint list views using filters  http://www.sharepointblogs.com/sam/archive/2007/10/16/how-to-restrict-public-access-to-sharepoint-list-views-using-filters.aspx

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Have a look at the following web, which introduce a SharePoint Permission add-on, it maybe can solve your problem:

http://www.sharepointboost.com/permissionboost.html

Here is a brief instroduction:

Assign permissions to all items separated from list permission settings
Let ITEM-RELATED users or groups have permissions to each item
Assign read or edit permissions to columns