If you provide some data and use cases, then I think the chances of an app appearing would be drastically improved 🙂
The short answer to your question is that we don't currently have anything on the near-term roadmap to build an app for Exchange. However, we'd be happy to discuss with you in more detail if you can share use cases and other information that you would like to see. You can reach out to me if you'd prefer to discuss offline.
One big use case for a Splunk app for Exchange for us would be the ability to see who has deleted or archived mails within a specific exchange mailbox.
Splunk for Exchange, covering Exchange 2007 & Exchange 2010 over Windows Server 2003 and Windows Server 2008R2 is currently in beta and expecting to be released in August to Splunkbase. Get in touch with your friendly Splunk SE to get on the beta program if you are interested.