If your data center environment has an existing investment in Microsoft Virtual Server (MSVS) 2005, this article will help you decide why and how you might want to migrate.
Deciding to migrate to Hyper-V: It's important to first make a business and technical case for moving existing infrastructure to a new environment. If you have VMs that are running happily on MSVS, there's no immediate need to move them. MSVS is a supported platform, and it's free on machines that are running Windows Server 2003. With that said, don't expect any significant updates to MSVS. Hyper-V is Microsoft's platform for going-forward.
So what are good reasons to migrate? If you're planning to standardize on a platform, it can be helpful to migrate existing MSVS VMs to Hyper-V (especially if your MSVS environment is relatively small.) This will allow you to build expertise on Hyper-V and to simplify management. Note that some management tools, such as Microsoft System Center Virtual Machine Manager (MSCVMM) allow you to manage both MSVS and Hyper-V from within a single product.