Once you have worked on any system with multiple monitors going back to a single monitor is difficult. I myself have done web development on a triple monitor setup, 3 19″ LCD panels and now anything less is hard. Now moving into the virtualization space, you may think that the multiple monitor setup is dead, but truly there are actually more possibilities then ever before. With vmware tools installed, and 2 monitors you can drag files back and forth between not only monitors but completely different operating systems.

Here are the most common setups which are possible in a virtual world, in regards to multiple screens:
1) Multiple Monitors = Multiple Virtual Machines
1.a) User a single computer with multiple monitors to display multiple virtual machines, either in VMware workstation or VMware Server.
1.b) Use multiple physical computers to remotely connect to many virtual machines on a virtual server.

2) Spanning all the monitors on one pc with a single virtual machine
2.a) Using VMware Workstation 6 or greater to span the physical monitors on a single system.
2.b) Using Microsoft remote desktop to connect to a remote virtual machine, and using the remote desktop spanning capabilities to cover all monitors

3) Connect to virtual monitors on a virtual machine from a physical machine
3.a) Using a tool like VNC, connect to a machine, and display one of its virtual monitors remotely, useful when you have multiple computers, each with a single monitor, as the mouse can seamlessly move between any monitor.

2 Programs Which Every Multi Monitor Setup Should Have:

Ultramon
Realtime Soft UltraMon
UltraMon is a utility for multi-monitor systems, designed to increase productivity and unlock the full potential of multiple monitors.
* efficiently move windows and maximize windows across the desktop
* manage more applications with the Smart Taskbar
* control application positioning with UltraMon Shortcuts
* multi-monitor support for desktop wallpapers and screen savers
* mirror your main monitor to secondary monitors for a presentation

UPDATE: You only need a single license for the base workstation, not each Virtual Machine
I have confirmed with Christian Studer of Realtime Software, that “you would only need a single license, the virtual machines don’t require a separate license.” So if you have a license for your main system you are all set for licensing.

Splitview
SplitView - Split screen monitor
Split your monitor! SplitView increases productivity by making it easy to work with two or more applications side by side. It helps make full use of your high resolution monitor and gives the benefit of dual-monitors without their associated cost. SplitView is perfect for deploying Citrix and Remote Desktop with dual-monitors, and ideal for using dual-monitors in ’span’ mode.

Sources:
VMware Communities: Rotated Monitor in XP Pro
Breaking News–Workstation 6 Previews VMware’s Future Server Virtualization

Ultramon forum
Realtime Soft Forum
Realtime Soft Forum

VMware Workstation 6.0 Users Manual [PDF format]
http://www.vmware.com/pdf/ws6_manual.pdf

Virtual PC Related:
Using Virtual PC with Multiple Monitors, Sort Of

Terminal Server Client
Terminal Services Team Blog : Multi Monitor support in the Vista TS Client