By now you probably know that you can only see items in SharePoint that you have access to. So security trimming your lists is pretty much built in.

The problem with assigning permissions to items so they won't be visible is that it takes a lot of work. It usually means that you need to break role inhertiance at some point and assigned the required permissions.

If possible you want to stay away from itemized permissions, specially if what you are really trying to do is just filter based on interest. MOSS has the Audience feature that does just that, however you still end up having to assign the Audience filter manually. Too much work!

I had a scenario where the tasks of a workflow were assigned to different groups. The users only wanted to see the tasks assigned to the groups they belonged to. Unfortunately you can't create a view to do this through the interface. But you can easily do it with CAML.

The WHERE sample below will only display items that have been assigned to one of the groups of the current user.

If you are not a developer you can still customize a view to use this clause with SharePoint Manager.

The where clause is using the Membership Element which can be configured to filter different membership types. In this case we are using the type CurrentUserGroups.