Opera just released Beta 2 of Opera Mini 4 mobile web browser which looks very promising, and they say that full version is not very far away. Opera Mini 4 includes the same (X)HTML, CSS, XSLT and XPath support as standalone desktop Opera browser. Majority of JavaScript functions are supported, except those that require asynchronous operations or user interaction once the page has been loaded (ajax). For the ones who don't know yet, web pages are rendered on the Opera Mini servers, which compile the web page into a compressed binary that is sent to the Opera Mini client. This reduces the data traffic sent to the phone, reducing users' data bills (many users still pay by the KB,) and reducing the time taken to display the page.
Developers, be sure to check out great article on dev.opera website - Evolving the Internet on your mobile phone.