In addition to the carrier restrictions, you also have device compatibility problems across handset manufacturers. I know a company that does J2ME development and they test on over 250 handsets.
That's why my startup chose to use the built in browser on the phone.
Does that help, though? I would be worried that the built in browsers all have varying capabilities, too.
Almost all the browsers are XHTML1.0 compatible (with the OpenWAP DTD). The only problem we've encountered are font size issues across phones.