Nikon. Nikon's D3X and D3S pro cameras are weather-sealed and can handle tough shooting conditions. The D700, D300S and D7000 have some weather sealing, but probably shouldn't be used in a downpour. The D5100 and models below it in the lineup aren't weather-sealed. Most of Nikon's higher-end AF-S Nikkor lenses feature enhanced or professional-grade dust and moisture resistance—as always, check the instruction manual for the lens before using it in harsh conditions.
Olympus. Olympus' pro DSLRs (the current E-5, and earlier E-3 and E-1) are weather-resistant, as are the SHG (Super High-Grade) and HG (High-Grade) lenses. These currently number more than a dozen, from an 8mm fisheye and a 7-14mm zoom through a 90-250mm zoom. (The Four Thirds System sensor's 2x focal-length factor means these frame like 14-500mm lenses on a 35mm camera.)
Panasonic. None of Panasonic's mirrorless, interchangeable-lens cameras (GH2, G3, GF3, GX1, etc.) is weatherproof, nor are any of the lenses for them. Panasonic does offer waterproof compact cameras. Use protection when shooting in wet conditions.
Pentax. The flagship K-5 DSLR is weather-, dust- and cold-resistant. We used our K-5 test camera in heavy rains on numerous occasions with no problems. Note that the new Q mirrorless camera isn't weatherproof, nor is the K-r entry-level DSLR. Pentax offers 10 weather-resistant WR lenses.
Samsung. None of Samsung's NX-series mirrorless, interchangeable-lens cameras is weatherproof, nor are the lenses for them. Use protection when shooting in wet conditions.
Sigma. Sigma's flagship SD1 DSLR is "splash-proof," while the SD15 is not, nor are the DP-series large-sensor compacts. None of Sigma's current lenses is weatherproof.
Sony. Sony's new SLT-A77 is moisture-resistant, as are its optional VG-C77AM vertical grip and HVL-F45AM flash unit. The DT 16-50mm ƒ/2.8 SSM kit zoom is also weather-sealed. Other Sony DSLRs, NEX mirrorless cameras and lenses aren't weatherproof and should be used with protection in harsh conditions.
Ewa-Marine (R.T.S. Inc.)