You should be able to buy an Apple TV at your local Apple Store very soon (I imagine if they don't have them today they'll be getting them over the weekend). What if you are one of the many folks who do not live near an Apple Store? According to CNet Best Buy has worked out a deal to get Apple TVs this Tuesday, a full 2 weeks before other retail outlets. Apple is only shipping them 3000 units, which will be spread out to over 800 Best Buy locations, but if you are jonesing for an Apple TV you might have to 'turn on the fun' at Best Buy.
Update: It looks like Best Buy might have been overly zealous with the whole 'exclusive' claim. They are still getting about 3000 Apple TVs though, so check it out if you're in the market.