It's going to be enforced more, as states look for revenue and the internet economy grows.
States have now formed the
Streamlined Sales Tax Project. There are 23 member states now, I think.
The goal of that project is to develop uniform guidelines, forms and collection procedures to lower the burden on businesses like Amazon as they move towards forcing them to collect sales tax. So, internet retailers would be collecting and remitting sales tax on behalf of states and hundreds of taxing authorities in the states.
The movement is on, though, and you'll see more and more 'net retailers collecting sales tax based on where the product is delivered.