Starting with VMware Fusion 4, personal users only need one license to install on all the Macs you own. If you're a business or educational institution you'll need one license for each copy of VMware Fusion. The relevant excerpt from the VMware Fusion 4.0 license agreement says:
You may install and use the Software for personal, non-commercial use on any Apple-branded products running Mac OS X (“Mac Computer”) that you own or control.
If you are a commercial enterprise or educational institution, you may install and use the Software on a number of Apple-branded products running Mac OS X (“Mac Computer”) that you own or control equal to the number of licenses purchased. Installing and using the Software on multiple Mac Computers with a single license is prohibited even if the computers are not running the Software concurrently.
VMware Fusion 4 upgrades are available now for a promotional price of $49.99. New users can take advantage of the same promotional pricing until the end of December 2011.