I'd call this one a bug. Either that or there's something that I'm just not seeing.
If you use the Flash CS3 IDE to create nested movie clips and set the Tab index property on those nested clips using the accessibility panel, the results are invisible to actionscript. Or, to say it another way - a child of a MovieClip will always show the default value for tabIndex if you check it with actionscript unless you set the value with actionscript.
If you use actionscript to set tabIndex, you seem to be able to check it just fine. More