The Atlantic isn't focused on writing, but it's the most fun to read; Publisher's Weekly has the best stuff on the industry proper, particularly the part of it in New York; Writer's Digest has the best stuff on magazine writing and probably the widest range of comprehensible stuff on the business end of writing; Poets & Writers has the best "craft-related" advice for folks who write poetry and fiction, but doesn't have much to offer me as a nonfiction writer. Cheers,
Tom's got it right. The Atlantic isn't necessarily a magazine about writing, but it features some of the best writing available in a magazine today. I read it nearly religiously. While it does feature some poetry, as well as a work of short fiction each month, it generally publishes works by known comodities, rather than those by writers who are just starting out. It also features great nonfiction writing. The current issue has a great piece--the first installment of three--on the recovery effort at the World Trade Center. I pick up a copy of Writer's Digest every now and then, if there happens to be an article of interest to me.