WAP Forum specifications determine which features are mandatory and which are optional. Support for XHTML Mobile Profile is mandatory in WAP 2.0.
Naturally each manufacturer decides themselves what they will implement into their devices. Not implementing XHTML Mobile Profile would just mean that such a manufacturer can not claim to have WAP 2.0 compliant devices.
But XHTML is the future language for both mobile and PC worlds, so it is highly likely that most of the devices will support XHTML Mobile Profile. And already several companies have announced their support for XHTML Mobile Profile as XHTML as a technology enabler (and WAP Forum) are part of Open Mobile Alliance. For more information about OMA please see http://www.openmobilealliance.org/