The hard part is getting a cable with the A sense, however this adapter is a good place to start:

(Links adjusted to use ASINs so should work)

http://www.amazon.com/gp/product/B000Y032X8

They also have a power adapter (the ultra-tiny to something much more common):

http://www.amazon.com/gp/product/B0009RFX24

(Note handling is charged once, so it is cheaper to get multiples to save on shipping if you can get them from the same place).

The above cable adapts the micro-B to the far more common mini-b and is small enough to carry with you.

For host mode, a mini-b to female-a is useful:

[url]http://www.amazon.com/gp/product/B000Y6UL5E

To make the micro-mini adapter cable into a host mode, carefully strip off the outer sheath and the metal shielding between the connectors. This will expose 5 wires - the usual red/green/white/black for USB, and brown for the host/perpheral sense.

Strip the insulation from the black and brown wires in the middle (carefully, using a lighter will melt - try not to burn - the insulation making it easy to strip).

Short the wires (and solder them!), and you now have a host mode cable. If you are ambitious , add a switch between the two wires so it can be used either way.

I didn't redo the shielding, but did use electrical tape over the wires.

Most "keychain" flash drives will work without external power. I keep a microSD in adapter in the female-a slot on my adapter chain so I can easily copy files betwen a PC and the n810 without having to worry about plugging in the n810 itself.

It will enumerate (recognize and use) hubs which can provide lots of power. You can even use that power to charge the N810 using the other adapter cable. It will give a "Hub not supported" notification. Then you can plug nearly any disk into the hub and it will mount. (you might need ext2/3 for Linux drives, it doesn't see mac partitions, but will recognize FAT iPods - will someone port GtkPod?).

Compiling more kernel modules adds more options. sr_mod.ko and cdrom.ko will allow you to access CD/DVD drives, but you will need to mount them manually. isofs.mod and ufs.mod will be needed to see files in most cases.

My EVDO modem (a UM-150) is recognized as is a serial adapter, both come up as /dev/ttyACM0, but there seems to be an interrupt problem with the cdc_acm.ko module - the serial adapter works (far better in a powered hub), but the UM-150 (requires powered hub) has problems with repeated characters or chatter. The UM-150's microSD is recognized without any problem though.