check the document "XHTML_Guidelines.pdf" in Browsing/WAP -> Documents! At least you should start your document with correct XML DOCTYPE definition. I've successfully used content-type "text/html" for compability issues. Try also run your document through W3 validator.
Please notice that Nokia 3510i has only WML browser and it does not support XHTML MP. So, you cannot access XHTML services with that particular phone or with Nokia 3510i Content Authoring SDK. Please see Nokia GSM WAP Phone Characteristics document (http://www.forum.nokia.com/documents) to find out which Nokia phones have XHTML browser.