Not sure what you mean by "sort" exactly", but no, the message stack is currently hard coded into voice mail, email and fax stacks (broken up by urgent and non urgent priority). You can not sort to get messages from a particular person or the like.
Unity 4.0 later this year will have a "FlexStack" option that allows you to arrange 8 stacks (voice, fax, email, receipts, urgent voice, urgent fax, urgent email, urgent receipts) into whatever custom order you want on a per subscriber basis and will allow you to jump around stacks directly over the phone. Not sure if that's what you're looking for or not...
Jeff Lindborg
Unity Technical Lead/Answer Monkey
Cisco Systems
lindborg@cisco.com
http://www.AnswerMonkey.net (new page for Unity support tools and scripts)