If there will be some PCs with Windows 10 pre-installed, it can only be few. I mean, I don't know how Dell, Lenovo, etc. would manage to create, test, and then pre-install their customized image on literally millions of devices within less than two weeks time. I don't think any manufacturer has the capacity to do it. I'd guess it'll take them a month a least. Plus, all these PCs are built overseas, and it's going to take weeks already to ship them to the US/EU. You might be able to pre-order your PC on the web, but I think actual delivery won't take place before the end of August. But, well, that's just my point of view.
They aren't allowed to sell them before the 29th or they would..shipping them overnight on the 29th is the best they can do and I'll bet you can walk into your favorite MS store and buy one on the 29th too.
Its a sweet spot for RTM its had very positive feedback internally. (but its not gold RTM) which means its probably release worthy but following updates to come.