Citrix Xen 6.5 should be out Q3 this year, possible by the end of August.
Rather than me repeating what is already all over the net I thought I would post a few links to sites with I think have some useful information on what is coming.
http://support.citrix.com/proddocs/topic/xenapp-w2k8-tp/xenapp-about-tp.html
http://neil.spellings.net/2011/06/28/notable-changes-in-xenapp-6-5-iron-cove/
http://zenapp.blogspot.com/2011/07/xenapp-65-tech-preview.html