The major weakness of the iPhone 3GS is it short-lived battery performance, especially when using its 3G connectivity. But this X-Power1 could be helpful with its in-line battery that charges up to give you 4.5 hours' worth of extra talk time. It's available in 6-inch, 20-inch and 30-inch lengths, or choose a retractable cord that could be easier to tote around everywhere.
Whether this handy $30 charging cable can deliver that 4.5 hours of extra talk time is yet to be determined, but it certainly would be convenient to have some extra power on hand. Our only problem is the goofy look of that dangling USB cable as you're using it for auxiliary power on that iPhone or iPod. Might want to just go with one of those Mophie Juice Pack Air cases instead.