Purchasing rMBP now - will come loaded with Mountain Lion?
Order now. You'll get Mountain Lion free no matter what, and it's a pretty painless process to install, particularly on a clean computer.

It's probably a toss-up whether you'll get Moutain Lion or not. It seems likely that it will be released before the end of the month, but when that image gets pushed to machines is anyone's guess. And even if you do end up with Lion, that may be a blessing in that you can wait out any 1.0 bugs if they pop up.

Ordering now gets you in the queue, and there's virtually no downside to not having a copy pre-installed.