Zend recently announced the "Zend Unlimited Subscription" for its PHP product line as a way to address questions of licensing when the number of running instances of software is highly variable - as it is in the Cloud.
Both Virtualization and Cloud Computing technologies pose significant challenges to traditional software licensing schemes (e.g. one license per user, one license per CPU or server). Both technologies create situations where the number of instances of any given piece of software can vary, and the spawning of new instance might cause a licensing violation if sufficient licenses are not "on hand."
This is not a new problem. InfoQ reported, in 2008, Microsoft's initial effort to clarify licensing of its products in virtualized environments. Others have raised the need to rethink software licensing because of dramatic changes in multi-core and hypervisor technologies as early as 2006. SaaS and Open Source licensing are alternatives to vendor solutions for variable software licensing problems.
The Zend announcement was for licensing of its PHP product line that includes an application server (Zend Server), a multi-server application manager (Zend Server Cluster Manager), and a PHP IDE (Zend Studio). Support for these products also has an unlimited license option. The importance of this announcement is proportional to the degree to which PHP has become a widespread standard for server side applications. Zend characterizes itself as "The PHP Company" and the unlimited licensing option is expected to provide a competitive advantage when companies adopt PHP for server-side Web applications.
Licensing agreements are almost always subject to negotiation and it would be interesting to know what kinds of solutions your company has explored as they move into Cloud and virtualized data center situations.