Thanks for reply. A couple responses:

1) OK, I understand about not being able to have dual slots, oh well. Right now full sized SDXC is not that much cheaper than micro, at 64gb ($75 for micro, $55 for full size, at adorama.com). 32gb micro is just $20-25 at newegg.com. 128gb micro won't exist for a while, 128gb full size about $155 (adorama.com), I have to expect 256gb full size will come pretty soon. So full size is preferable but micro is definitely acceptable in my opinion.

2) Re flat batteries, the idea is to use decent ones intended as components ( http://www.sparkfun.com/products/8483 , http://powerstream.com/li-pol.htm ), not some mislabelled phone battery. They are standard sizes and shouldn't become unavailable. I don't mean use a custom pack from some consumer device. For putting two in the same plane, just pick sizes that fit the case, they don't have to be BL-4J's. I've always heard lithiums in parallel work very well, for both usage and charging, even if capacity is unequal. For example, the Cowon X5L audio player uses two lipo cells of different sizes.

3) Lipo seems to be safer than li ion (scary 18650 explosion pics: http://www.candlepowerforums.com/vb/...ead.php?262234 ). Ouch. And I think supplies of batteries for currently popular phones or digicams aren't likely to dry up.

4) Lens mounting thing: just a threaded hole in front of the camera is fine. Exact distance from lens isn't critical, as the camera's focusing mechanism does the right thing. People do this all the time with digicam to binocular adapters and so forth. I haven't done it with a phone but have done it with digicams. There are all kinds of iphone lens accessories at dealextreme, this is the same idea.

5) NiMH works great, those problems aren't nearly as bad as you describe, and it's easy to replace them if they go bad. Plus in a pinch you can use regular AA alkalines, very useful. So I still would really like an option like this, at least as an add-on. (It would be enough to just have some tapped mounting holes in the case, to attach a battery pack or other accessory to).

6) If you really do use 18650's then I guess you could fit 2 or 3 of them, very high capacity. Maybe the best approach is just have a single battery, thin case, with mounting holes for an add-on pack. The add-on pack would be pretty simple so there could be several sizes (2x18650, 2x BL-4J stacked, 3x AAA, etc) or fancier versions could have their own charge circuits (5V input). The packs could probably be 3D printed plastic.

Note that the voltage of the Nokia 2mm charger is somewhat higher than 5V, as you're probably aware.

One other idea: a powered USB host port and microhub (using existing USB2go) would really be awesome, if feasible. Then maybe there could be an internal space for a second microsdhc card on one of those tiny flat USB adapters.

PS, yes I can send $50 for pre-order of back cover, let's see how that goes before doing whole case.