I found this one here in the atlassian forums, unfortunately the link posted there is broken.
We are announcing our end of life of Atlassian support for Internet Explorer 6 on JIRA.
This will be effective from the launch date of JIRA 4.2 (target Q3, 2010). This means that JIRA 4.1 will be the last version of JIRA to support IE6. (From JIRA 4.0 to JIRA 4.1, all of the main functionality will work in IE 6; however, some of the visual effects will be missing).