Tomorrow, Tuesday 20th Feb, from from 2:00AM - 5:00 AM (PT), Zune Marketplace will be offline for maintenance. It's maintenance only, no firmware updates/new features.
You'll still be able to access the catalog and browse music, but won't be able to buy music, create new accounts, manage your existing accounts, purchase additional points, or view your account history. And the account management section of Zune.net will also be down. You'll be able to access the rest of Zune.net without difficulty. Take care!