Your options are most likely limited even among the upper end of the name brand docks I've seen. Charging likely will always require AC power. I'd start looking at the Logitech models, which have pretty good sound IME and O. Again, not sure about battery recharging of your iPhone though. You might require a dedicated solar device for that. Keep in mind regardless of what you end up with, the sound quality *outdoors* with such a relatively underpowered device always pales by comparison to the performance indoors since the depth/bass/dynamic range is lost in the large open space with no reflective surfaces to make up for the lightweight power output and petite speakers.

Logitech pure-fi anywhere 2 would give close to 10 hours on the rechargeable battery though.

http://www.amazon.com/Logitech-Pure-Fi-Anywhere-Compact-Speakers/dp/B001G5Z82I/ref=sr_1_1?ie=UTF8&s=electronics&qid=1274303148&sr=1-1