Two options for BlackBerry that I've come across:
OtterBox makes has a very well made (though bulky) case for the 8800 (and Pearl, 8700 and 7200). While not fully waterproof, these are certainly highly water and dust resistant and provide great protection for BlackBerrys in extreme environments. My only criticism is that the hard part of the case creates a ridge just above the trackball, making use of the trackball and the keys to the left and right of it slightly harder to use. Otterbox LINK and BlackBerry Cool LINK.
Another set of nice cases are available from Seido (LINK). If you really have money to blow, buy an OtterBox and combine its keyboard membrane with the Seido Hybrid Crystal Case (LINK) to make a sleek, water and dust resistant case without the bulk (but of course less of the protection) of the OtterBox.