W7 creates that secret boot partition if you let it do the formatting itself. Because you've pre-defined the space, it won't happen.
On my setup W7 was added to the 2nd HDD, so I disconnected the 1st to protect the HnS boot.
That meant I had two independent BCDs, Vista's and W7's on different HDD, neither aware of the other, and I just manually edited the menu.lst to chain to either one, as you can see (created the little empty disk identifier (Windows7.I.HnS) manually too.
If you're adding W7 to the same disk as Vista, you won't be able to do it the way I did.
The second longhorn will add itself into the BCD of the first, and it might replace bootmgr. (which of course is not really bootmgr under HnS - Vista's bootmgr has been renamed to bootmgr.HnS and the thing pretending to be bootmgr is really HnS's grub)
So adding W7 might break HnS.
Safest to remove HnS (run the UI and ask it to remove itself), Install W7, run the UI again and define both longhorn systems as "Vista".
Don't run XP until you've got HnS back in control to protect all your restore points.
You'll get a double menu at boot, HnS then Vista/W7 where you'll choose between the two longhorns.
(that's the advantage of my technique - I still retain a single boot menu by chaining to 2 separate BCDs)