My take on this thread, would be to get a few pcb protected lipo's, a dc powered lipo charger, and a sepic (single ended primary inductive converter) to get a constant voltage (with variable wattage/amps) to always squeeze every watt of power out of the dynamo when its kicking out lower voltages. albeit at an a effeciency loss to get the current produced at lower dynamo voltages, but the energy would be wasted anyway so its a net gain as far as dc/dc sepic conversion effeciency.

Thats how i charge my vlra's using a 20w solar panel, since my sg-4 morningstar won't use the wattage produced below ~14v to charge the batterys. The sepic takes a voltage as low as ~3v depending on the IC/board, and boosts it up to whats needed, or bucks it down if the voltage is too high, though the charger can handle well above the panel max voltage (21v), that may not be the case with a dc powered lipo charger expecting 12-15v like the tenergy one i have.

Use that to charge ~25wh of lipos with a dc powered charger and a sepic to get enough volts (at the expense of reduced amperage/watts, but its better than wasting the power produced), stuffed into a waterproof SS waterbottle with the charger and sepic.

Then just use either a 5v regulator, or another sepic to charge your n900 from a female (water resistant) lead coming out of the bottle.

SEPIC dc/dc converters are pretty neat for getting every last watt out of low voltage sources, by boosting them up to whatever voltage is necessary to charge. Great for dynamo's and solar panels when they can't kick out enough volts to power the charger, though you lose a few % in effeciency lossses, i'd wager theres a net gain from the otherwise wasted low voltage power boosted to whats needed. And you get overvoltage protection, since any voltage above what the sepics set at, is buck'd down at a higher amperage.

Joule thief circuits might be worth looking into as well, to get enough volts when your source voltage is too low to power the charger in question. You'd want to implement some kind of voltage regulation though, sepic boards capable of 10w are cheap on ebay though, i'd just buy one of them. If you know what your doing though, designing your own voltage regulator/buck/boost/current limiting board, is fun too.

Just don't use a non-regulated joule thief as your usb output supply. Theres nothing to stop overvoltage damage to your usb devices, assuming your dynamo can pump out high voltages. But using a zener diode, a resistor, a transistor, possibly a few caps, and a inductor, its a cheap way to diy a boost dc converter.

And then just hook up one of those 5v 3a regulators/another sepic to a female usb jack with the center two pins shorted, and connect that to your phone.

I built a 6 cell usb nimh charger with a sepic, and it works great for a simple battery pack for my n900. My current project is using lipo's instead of nimh's in a similar application.

Another thing to take into account, is you may want a simple rf low pass filter, since i would assume power coming from a dynamo/dc motor/altenator is fairly dirty frequency wise, and possibly even ac depending on your dynamo/generator. But most voltage converter boards already have the circuitry to accomplish that, and its easy/cheap/a better option to just spend a few bucks on a sepic regulator.