It takes a community (to service mobile devices)
Volantis is relying on a community to build out its device support. This is the right model for mobile open source.
Volantis just released its Mobility Server under the GPLv3 license, which should go a long way toward helping to grow Volantis' community further. As and open-source mobile projects, it's hard to conceive how any proprietary software company can compete with open source in mobile.
It's not a question of software. Anyone can write that. Rather, it's a question of keeping pace with device proliferation, as OStatic suggests:
Volantis had already made its Mobility Server available as a free download in late 2007. By open sourcing it, the company is looking to the broad development community to help deliver web sites and applications aimed at mobile users for delivery on an ever-increasing range of mobile devices.
Currently, Mobility Server is tuned to deliver content to more than 5,000 types of mobile devices, when it has historically been difficult for developers of open source mobile technologies to reach so many hardware types.
The mobile market is still the province of a relative few high priests (Vodafone, Nokia, etc.) that have jealously guarded their secrets. With the iPhone, however, it's hoped that this market will open up somewhat.
Volantis makes creation and delivery of mobile content easy. Perhaps now the market is finally ripe to receive it.
Disclosure: I am an advisor to Volantis.