There is a brief discussion on
wowwiki of the ten action bars blizz makes available in the game, all of which are available in nUI 5. Basically, bars 1 thru 6 are on the scrollable "top bar" in nUI, bars 2-6 also appear elsewhere in the nUI dashboard, although I can't remember offhand which is which. Bar 10, which as far as blizz is concerned is "unassigned" is also permanently available on the nUI dashboard (so seven of the ten bars can always be visible). What's not always visible are bars 7, 8, and 9, which blizz (and nUI) use for various warrior stances, druid forms, or rogue stealth. Logically, since a cat-form druid, when prowling, has abilities not available to him otherwise (pounce, for example), there should be a fourth "form bar" which would automatically replace bar one when the druid prowls. I guess blizz decided that having bar 10 "unassigned" was more important.
As Xrystal says, I think Spiel plans to include some way (more bars?) in nUI 6 to auto-change bar 1 when prowling in cat form.