Mobile devices that support WAP 2 will most likely support a number of markups.
WAP 2 devices are required to support XHTML-MP and WML 2.0. Supporting WML 1.x is not a requirement, but most devices will in facts support it, for backwards compatibility with all the existing WAP sites.
WML 2.0 is basically a revision of WML 1.3 and is not effectively used in real-world sites.
To start developing WAP 2 sites you should probably start reading these:
More topics are covered on this FAQ, of course.