Yes, you are right, ie 6 and ie 7 have a buggy z-index implementation.
Anyway that article shows that ie 8 behaves correctly in every z-index test but tabPanel problem still shows up in ie 8 browser.
I found a workaround:
using style="position: relative; z-index: 10000;" on the first tabPanel solves the problem!
I tested it on ie 7 and ie 8.