Formatting is permissible and a WYSIWYG editor is on the list of things to do here on foolishmortal.org.
The spec indicates that messagses should be in XHTML and adds a few namespaced element types to handle some customization. http://socknet.net/w/Socknet_Message_Format
When messages are shown, they are filtered for specific elements, attributes, and certain attribute values. Such as <style> tags and style attributes. (Actually style tags are so useful that we may have to dive into them and just get rid of the bad parts.)
Allowing for bbcode is not a bad idea and it would be as simple as converting that bbcode into HTML when sending it to friends. It may require that messages be stored in both bbcode and HTML format so that they can be easily edited later.