They don't really have a history of doing that. Greenhopper was acquired and integrated into JIRA as JIRA Agile. Hipchat was acquired and now it lives on and nicely integrates with nearly all Atlassian products (JIRA, Bamboo, Confluence, etc).

My guess is they'll keep Trello running for a while as-is, and slowly rework it to become "JIRA Lite". It will likely have Trello-like functionality with JIRA branding, and some sort of migration path to the heavier JIRA for companies that have grown a lot.