Sony Ericsson has decided web applications are the way of the future, with a new SDK and emulators enabling the development of cross-platform applications.
Like many of the non-aligned manufacturers Sony Ericsson is forced to support application development on a variety of platforms, including Android and Symbian, so shifting developers into AJAX makes sense if one can extend the platform to suit mobility. This is something Sony Ericsson achieves by integrating the open-source PhoneGap platform.
If you want to have device-resident AJAX applications with access to phone functions then you have to extend the platform, which can be done using the OMTP's BONDI initiative, the W3C's forthcoming widget standard, or the open-source PhoneGap platform.
Over time these will likely merge, both the API and security model, but until then small and irritating differences will make life more interesting for developers.
Sony Ericsson is endorsing PhoneGap with a load of tools including emulators and development tools. Right now it's only the Xperia X10 and Satio that are supported but the company clearly has high hopes for web-based applications - mainly that they render the choice of OS irrelevant. ®