M-Edge has a waterproof case for K2 (now marked down to $39.99 from the original $79.99 I see...); I assume they'll have one for K3 before too long.
http://www.medgestore.com/products/kindle2-guardian.psp
I thought the K2 design was deficient in several respects, however: it used a clamshell design (with o-ring seal going the entire circumference of the case) rather than one that opened at one end to allow Kindle to be slid in (requiring a much shorter o-ring and smaller lid with less potential for warping/leaking). They only claim it to be waterproof to 1m depth, which is not much of a margin to work with and rules out its use for reading-while-scuba-diving (for example <g>). Also there was no way to wake the Kindle from sleep mode without opening the case, though I've since learned of a series of commands you can enter at the Home screen that lets you turn off the screensaver, and which would circumvent this limitation.
Hopefully they'll improve on this with the K3 case, and stick with the lower price.