They stated they'd test server loads, but put a cap on it. They have yet to state how they plan to do this. Whether it be an open download and then just first come first serve on the connections. Meaning they set a cap on the server load at X. If the X+1 person tries to log on, they get a server full message. This is speculation on my part, but from what I read earlier, I think that might be how they're gonna do it.
Remember, open betas are used for two reasons. To stress test the servers and to get promotion for the product. They've got the promo part down, so they just need to stress the servers. As to why this can be done after going gold? More than likely any fixes on the server load will be done on the server software, not the client software. If it is client related, that's what auto-patching is for. There have been days were Ultima Online had 3 patches in a day for their client software.
It would be nice if there was a download to try the game offline to test out the flight model (or armor model for those types.) As it is, I'm lucky in that a friend has pre-ordered it. I'll just go fly on his account to see if it's worth splitting time with AH.
As for June 6th... uhmm, it's not the Pearl Harbor movie they're hitting at there... Though doesn't make much sense that their initial campaign is the 1940 Blitzkrieg if they're trying to take advantage of D-Day.

I guess it all works out tho, the Pearl Harbor movie, Enemy at the Gate was just out, and D-Day release...