Supportability and licensing are key when it comes to virtualization a BizTalk Server 2010 production or any other environment.
Similar to SQL Server Enterprise, BizTalk Server 2010 ENT can be licensed for unlimited virtualized processors that are available on a single physical server. As customer you will be required to license the number of physical processors on a server.
In case you desire to build your own BizTalk Server 2010 virtual machine for learning purposes you can use Hyper-V in case you have Windows 2008 R2 with Hyper-V to your disposal, or an alternative like VMWare or VirtualBox. The latter are appropriate in case you want to build a virtual machine on a laptop or desktop with a client OS like Vista or Windows 7. You can download the ready made VHD, yet you have to convert them either to VMDK or VDI format.