Yes, you can access the mobile web site from a range different mobile devices, but the platform, I suppose, is still the web server and the client browser (the latter ideally being the "same" on any device).
When talking mobile and cross platform I am thinking frameworks such as PhoneGap and Appcelerator Titanium, but still with a reluctance to call it cross platform as the deployed products are not the same. I guess I am hung up on the idea that it has to be directly equivalent to the Java virtual machine to call anything cross platform, but maybe I am the one who is misguided.
What is the general opinion?