Thanks for the feedback so far everyone! Yabbas, a few points to discuss:
1) You say that you're injecting 5V onto the USB no problem; if you are simply adding 5V to the USB power lines I'm quite surprised that your devices are working without software hacks/modifications. The reason that you need a powered hub/intelligent device is that the USB standard dictates that when you connect a device, it sends a message to the host requesting a certain amount of power be supplied; the host (in this case the tablet) then responds by either supplying the power, or disabling the device if sufficient power is not available. If you just do a direct injection the tablet still thinks it's only capable of supplying it's limited 125mA, so even though the device technically has sufficient power through your external source, the tablet's kernel SHOULD refuse to enable it. Other members of the forums here (along with myself) have done experiments in the past to confirm this, and that the only way around it is to forcibly disable USB power management in the kernel, which at the end of the day really isn't a good plan. I don't remember the exact thread, but the discussion comes up about once a week whenever somebody asks about "my USB device needs more power!".
Also you're talking about cost... right now you're saying 10 quid is quite expensive; to integrate a USB hub chip into the adapter would add the cost of the components (chip plus a few resistors and a cap or two, not a big deal.. say another few dollars tops) PLUS the cost of PCB production and integration (adds a few more dollars per unit). In addition the size of the adapter would have to increase to accomodate both the components required and the power port (the power port itself while it looks small on the outside actually takes a fair amount of space). So at the end of the day your basically doubling the cost of the adapter to add a power port to it; not only that, but since there's a hub chip in there now you've actually reduced the amount of power available for devices when if you're travelling around and don't have an external power source to plug in available (usb hubs consume approximately 80-100mA according to USB standards... again whether they really do or not the issue, it's how much the kernel says they do, and thus how much it assumes it has left to allocate to other devices).
Don't get me wrong, I'd love to have a solution like you're talking about, but I've been researching it for about 2 months now and at the end of the day it just doesn't make any sense. The power supplied by the tablets currently is sufficient for most "on the go" type of equipment (thumb drives, etc). With the exception of the extra card readers (and I know what you mean) most other USB devices are connected when you're stationary somewhere, so having a second small cable/adapter/your own USB hub whatever for those times is not a huge issue as you're not concerned about ultra mobility. A better bet is to try and convince Nokia to make a bit more power available on the USB bus from the tablet itself in future software/devices... (perhaps improvements in battery technology/power management can make this possible?)

On the 90 degree point... I think it's quite mute really; the adatper still has to be the width of the female A connector on the bottom anyways in order to fit the "360 swively hinge thing", and the male micro is actually quite short (it looks larger in the picture). In it's normal configuration it would stick out about 3/4", with a right angle it would be about 1/32" smaller, and you'd loose some of the benefit of the 360 as the hinge would be butted up against the tablet itself on one side.

I'm all for doing it right the first time (especially in this sort of quantity!) but it has to be viable. (Also if you're concerned over shipping costs... group buys or finding somebody/setting up your own local distribution for them would make shipping more afordable as I can send probably about 15-25 for that $11, and higher quantities aren't a lot more... it's just that's the minimum to ship anything overseas and have it actually get there).

Thanks again for all the ideas and feedback!
-Rob