Update 2 is already here and is double size vompare to the first one, here is some more informations :

This update resolves the network connectivity issues that affected customers with both IPv4 and IPv6 network interfaces enabled who installed Update 1. This issue is described in XenServer 5.5 Update 1 Upgrade Causes Virtual Machines to Lose Network Connectivity.

Additional Changes in Update 2
Addressed an issue where the system was falsely reporting lost access to the HA metadata disk.
Fixed issues upgrading from XenServer 5.0 to the roll-up ISO for XenServer 5.5.
In addition, the XenServer 5.5 documentation was rereleased to coincide with Update 2. While the changes in the documentation are not significant, there are some differences between XenServer 5.5 and XenServer 5.5, Update 2.

You can download it here : http://support.citrix.com/article/CTX124027

You can download the updated ISO files here : http://downloadns.citrix.com.edgesuite.net/4701/XenServer-5.5.0-Update2-install-cd.iso

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I read this "option" on Citrix's forums. I'm not sure what will happen with the next update with this action once it has been done.