I've got the exact same thing. Everything passes its tests, but refuses to connect to Live. I've done all the stupid things, resent HD, NAT is open, recovered Gamertag, upluged and reset everything, still doesn't work.
The odd thing, took my Xbox to buddy's place, and it worked flawlessly on his wireless, brought it back home, and doesn't work again. Which is odd, cause it worked just fine for the last 6 months.
When is Microsoft going to admit the problem is in their "update" and not on out end?