It seems the two biggest concerns about the n900 are battery life and scratching the screen so how about the a case that addresses both.

Does anyone think a case with a solar panel on the back that can trickle charge the phone at the same time would be great.

If pushing it I would ask the panel be on the front of the case and it could be opened at 45 degrees so you can still charge while tilted and a clip on the back for a belt mount that lets it sit flat.

The iPhone has a 3rd party accessory that already does that. It plugs into the bottom and both extends the life of the device, and protects it from harm. I don't know the name of it, but I saw it advertised on the web just recently.

The problem with that is where would it actually get enough sunlight for it to be worthwhile?

Personally, if I were out and about I would have it in my hands so it would have to work while I was holding it and I would have to sit with the sun facing me, which would probably make it so I can't see the screen as it would be in my eyes.

Then if I am not using it no way I would ever leave it anywhere where it would get enough sunlight, as that would mean leaving it away from me where it will get stolen.

This is exactly why I have never bothered to buy any of these solar devices in the past for my phone, as I cannot see a situation where it would actually work.

Personally, I think we need solar hats as that is the only way I can see it being worthwhile. Of course would likely lose our N900s due to being beaten up for looking a complete tit then.

I think that separate unit is more practical. It sits about 10h in the sun and then you can charge the phone.
Like this one that has 2000mAh battery inside.

I can see some peoples concern about leaving the phone but I guess it really depends on how you use your phone.

Mine spends most of its time sitting on a desk and will need to be in a case anyway, why not have the case trickle charge it at the same time.

and the solar power comes to the case from .... ?

I had a device like that, it would never charge.

It would charge fine from a mains adapter, but no matter how long I left it in sunlight it seemed to get very little charge. As such I am sceptical about any such devices now.

Such thing is useful for outdoor-sey people where they're away from power outlets for awhile and will be in open places under the sun. Otherwise, check out the (rechargeable) battery packs because they can pump the power (charge) faster and you don't need to have the gadget stationary\angled to the sun all the time.

Or maybe a more practical solution would be multiple spare batteries and an out of device charger?

I think those external battery packs probably are more practical as you could get some insane mAh rating one which would replace 2-3 spare batteries and not require powering off your phone to use it.

Like I said though, those solar ones I am dubious about as the one I had, despite being left in the window for weeks in direct optimal sunlight, never charged. Yet the charging led was always lit and the battery clearly worked as it charged fine from a mains adapter and also I later dismantled it and found the solar panels also worked fine.