Update: Paul Allen, whose yacht Octopus is currently keeping station near Mermaid, reported a half-hour ago that James Cameron is on his way back to daylight. So, for whatever reason, the bottom time on this first solo dive will be closer to three hours than the anticipated six hours. If both ascent weights slid out on command, he should be surfacing within 20 minutes.
All systems are go, reports James Cameron from the Challenger Deep. He touched down about three hours ago, which leaves him three more hours of exploration time in the mission plan. A throng of followers are following Facebook and Twitter feeds, accessible through the website.
That's enough time to work with the two landers, if they came down within a couple of miles of his touchdown point. With the caveat that the ships turned off their AIS transponders shortly before the launch, here's the flotilla's last reported position, on a Google Map. It should be pretty close to the center of activity.
This link at Deepsea Challenge is a sobering summary of the risks the team has had to think about, and overcome. Some stresses were well-tested in the lab, such as the effect of pressure on the steel pilot sphere, its viewport, and the point that is probably of most concern, the penetration plate (see posts below). Deepsea Challenger came with a generous margin of safety and the sphere did fine during its unmanned test, so it's hard to see much residual risk, pressure-wise.
Other external hazards can't be dealt with so precisely, such as entanglement. Entanglement seems to pose no risk during this mission. For one thing, the two landers apparently have no tethers to the surface, so that avoids the risk of fouling the sub's thrusters with such lines.
But if the sub is used to explore more locations in coming years, as is likely, giving a wide berth to any tangling hazard will be a part of every mission plan. An ultra-deepwater wreck -- say, some storm-wracked ship that happened to come down in the Challenger Deep years ago -- would be a tempting, but dangerous, location for investigation by a manned sub.
Meanwhile: so far, so good!