For virtualization on BizTalk Server 2010 supportability and licensing are key.
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.