When we ride, we always carry our iPhone in a jersey pocket. And that means we're in need of a way to keep sweat (and sometimes rain) from creeping into the electronics.
One solution we've found is the Smart Sleeves cover. We first found these at the California State Fair this year, and have been using them ever since. Smart Sleeves are made of a very clear plastic that lets you see the screen and use the touch controls while the phone is still in the bag (an improvement on other carriers that require you to remove the phone). The manufacturer claims that the bags are waterproof, although in practice we've never tried submerging the phone.
Yes, a less expensive ZipLoc bag will also work, but the Smart Sleeve allows easier use of the phone and can last much longer. More info: www.smartsleeves.com