Here's a good primer from the Washington Post Wonkblog on the legislation to make online merchants collect local sales taxes on purchases. Many are involved, but Walmart is pushing it to level the playing field with online merchants that don't have brick-and-mortar outposts in every state, as Walmart does. Amazon, to name the biggie, is on the other side.
Senate passage looks probable. In the House, where Rep. Steve Womack is, naturally, carrying Walmart's suitcase, things are more problematic. Local governments are missing billions nationwide because online retailers are exempt. It punishes local businesses who collect the tax, big and small.