I am currently working on a large SharePoint project looking to migrate to MOSS 2007.  I have been involved in the investigations into the migration of our custom 2003 web parts over to MOSS.

One of my current challenges is to replace the top navigation bar with custom tabs, as our customer does not require the 'standard' set of tabs to be displayed.  Also the tabs are dynamic depending on what level of user is logged in and what responsibilities they have to the organisation.

So far I have found 2 strategies:

1. Add a custom web part to the page that will generate the custom tabs when the page is loaded.  This also requires the 'My Home', etc tabs to be removed via editing the master pages.  This is quite slow and increases the load time of the page.
2. Add a custom navigation provider to the master page.  This is quite involved but does have the advantage of fast load times for the page if implemented properly.

Will post more details later as I am in the middle of developing something else (non-sharepoint)