SQL Server 2012 now has an option for core licensing for Standard and Ent. You must buy 4 cores at a minimum and additional packs are a minimum of 2 cores.
Core licensing gets a little hairy for VMs. Each VM CPU is considered a core, and the 4 core minimum still applies. However, if you have Ent with Software Assurance and license all cores on the host then you can run unlimited VMs with SQL Server configured however you like.