See https://jira.atlassian.com/browse/CLOUD-6999

Excerpt: "Problem

With Cloud apps, domains are always of the form *.jira.com (for legacy Studio customers; no longer available) or *.atlassian.net (for any OnDemand/Cloud Customers). Customers are interested in the ability for them to use their own domain names, e.g. jira.example.com.[...]"

This issue has been open since July 08, 2011.

This would greatly benefit companies that would want to maximize Atlassian OnDemand services to appears seamlessly integrated with corporate branding.