After 4 weeks I've managed to be listed by the Open Directory Project, Lycos and FAST Search. hooray! Still no luck with Yahoo though

My question is do I ever need to resubmit to these sites or will they just keep me listed as long as I exist.

Also, I showed up in the Google searches for a about 2 weeks and now my listing seems to have disapeared. Is this normal? Should I resubmit?