Based on past Windows releases, Windows 7 has to RTM (Release to Manufacturing) by late August or early September to get pre-loaded on new PCs by the holiday season. I would expect major PC manufacturers to have preloaded Windows 7 machines in retail outlets by mid-November at the latest. In fact, Acer has already said they expect to ship their first preloaded Windows 7 machine in late October.
Will it be available at retail outlets in August? No. Microsoft will wait until the first wave of preloaded Windows 7 PCs are on the market (mid to late October) before releasing the retail (boxed) copies. However, if you're a member of MSDN or TechNet you'll get a copy of the RTM release.
You should also keep in mind, per Microsoft's policy, any PC sold after July 1 with Windows Vista will be automatically eligible for a "free" Windows 7 upgrade license. Just like last time, these "coupon programs" will be handled by individual PC manufacturers.
RTM = Microsoft's term for "Gold master" in the Mac world.