<p>What format do I use for Date/Time when writing to an XML file using .NET? Do I simply use <code>DateTime.ToString()</code>, or do I have to use a specific format?</p>

What does the DTD have to say?
If the XML file is for communicating with other existing software (e.g., SOAP), then check that software for what it expects.
If the XML file is for serialisation or communication with non-existing software (e.g., the one you're writing), you can define it.  In which case, I'd suggest something that is both easy to parse in your language(s) of choice, and easy to read for humans.  e.g., if your language (whether VB.NET or C#.NET or whatever) allows you to parse ISO dates (YYYY-MM-DD) easily, that's the one I'd suggest.