Oh good, I was looking for a place to prove myself.

"until" and "up until" include the time in question.

If I say, "I am going to work on my essay until Saturday," this means I am going to be working on the essay from now until some point on Saturday at which point I'll either stop or the scenario will change.

If I say "I'm showing you my plans until summer," this means I am showing you my plans from now into the summer, as in "before autumn starts this plan will change."

You are using the wrong definition of "until" simply to suit your purpose. The sentences in question aren't even in the negative. If he said "I'm not going to show you my plan until summer" it would mean he's not going to show you his plan before summer, but it also doesn't mean he's going to show you midnight of June 21, 2011. It means AT SOME TIME IN THE GDMN SUMMER.