Sharepoint ECB menu items not visible when doc lib is displayed in a web part

My document context menus display correctly when access开发者_如何转开发ed directly from a document library but when this same doc lib is displayed in a web part, the context menu is missing various items. I debugged the code in core.js using a userid that had all permissions turned on and noticed that the permissions mask was properly set (to 0x7ffffffffffffff) when the context menu was accessed directly from the doc lib but when it was accessed from the doc lib in a web part, the permissions mask was 0.

Is it necessary to permission users for web parts as well as for document libraries? What else could explain this behaviour?

We recently migrated to Sharepoint 2007 from Sharepoint 2003; this problem does not occur in our SP 2003 site, only in SP 2007.

Update 1: New info which may make some of the existing discussion obsolete: on some of our pages, we display doc libs using the webparts that SP automatically creates for you when you create a doc lib (SP does do this, right? I'm a bit new to SP so bear with me). The context menus for the docs displayed in this wepart work fine as our users have been permissioned for these doc libs and SP is giving the correct permissions mask.

However, we also display certain docs by iterating over the lists associated with a given web. We get our web like this:

private SPWeb _web;

And iterate over each list liks this:

foreach (SPList list in _web.Lists)

So, the only question I have now is, is it possible, necessary and/or required to assign permissions to SP lists as well as to doc libs?