Chris Hahn sent over this quick warning message for users of XenApp 6. A potential bug with 11.x and 12.x windows clients accessing a Citrix XenApp 6 server.
When logging, there is no logon progress dialog for Published Applications until the login process completes. For Published Desktops, there is just a black screen. More information in depth conversation on this topic can be found on the Citrix Forums.
Unfortunately according to Citrix, to fix this, both a new client (12.1), and server side patch is required which have not been released yet.*
*(As of 09/2010)