We've seen some good portable iPod/iPhone audio systems in our time, but Eton's Soulra is one of the more-intriguing products we've come across in this category.
The unit has a rubberized aluminum body that the company says is splash-proof and the speakers are sealed on the inside to provide "protection from sand, water, and other outdoor elements" (according to the company's press materials). On top of that, the $199 Soulra incorporates not only a rechargeable lithium ion battery but a "high-efficiency" solar panel that allows you to trickle-charge the unit while it's outside (so long as there's ample sunlight, of course).
The best thing to do is charge the battery fully using the included AC adapter before you take it outside. Fully charged, you should get about 4 hours of battery life from the system as your iPod or iPhone is charged in the cradle. But that number will rise if you flip up the solar panel, which doubles as a cover for the dock.
Recharging the unit via the solar panel doesn't happen as quickly as it does using the AC adapter. It takes about 10 hours to fully charge the unit in the sun versus 4 hours with the AC adapter (solar charging will not work through a window). But so long as you've started with a good charge, you should be able to leave the unit outside playing for a good portion of the day (around 6 hours).