Porn on handheld devices should continue to increase along with the marketshare of these types of devices. If a device is capable of pictures, video, sound and even text, people will view porn on it.
The trend has generaly been that handheld devices are a few generations behind desktop or settop devices of the same time period. So the more computing power we get for our computers, the more that trickles down to handheld devices. As handheld devices become more and more media capable, it becomes easier to view media, so media specifically produced for those devices is not as necessary.
Eventually it seems logical that virtually any media you own would be transferrable to any device you own (licensing debates aside). So porn acquired through any source (e.g. DVD, online) could be watched on your TV, on your computer or on your pocket video player. So theoretically the need for device specific media should diminish while the ability for people to consume media should increase.
A few years back handheld devices were also being hoped for as the next wave of Internet goldrush in being a "new" connected platform. It's tricky right now because delivering content directly to a handheld device, like a cellphone, is restricted by the providers (mostly). As the ability to transfer media across devices becomes easier, then the dependency on those systems for delivery should reduce.
Hopefully, ultimately, more viewing devices should equate to more sales of media (porn or otherwise) for everyone.
