One Watt at 12 V (I presume, since you have a car charger clipped to it)?

That would yield you 0.083amps. That's less than 1/10th of an amp. 1100mA less than what the N900 calls for.

1 watt/12 volts = 0.08333333 amps.

That solar panel has to have a higher rating than one watt.

How long it takes you to fully charge the n900?

if that one guy's speculations are correct, that does never get the battery fully charged (unless battery is charged while device is off)

This is gonna work? A little expensive but it is very convenient.

of course it depends how the car charger deals with the voltage. but even with plain regulator driving everything above 5v to resistor is good enough if 12V 6,5W is supplied. it is a bit better than charging via usb port when sunlight is optimal

I never had a constant supply of sunshine to thoroughly test this arrangement at length, but I would hope that it at least provides some power so the battery depletes less quickly.

The scenario I envisioned involved attaching the panel to a bicycle while on a 6-8 hour daily ride. The N900 would be used for music and GPS, when the latter will be fully functional

What's wrong with the PowerMonkey eXplorer?...

Works perfectly, used it whilst travelling earlier this year.