VMware supports an in-place upgrade of vCenter Server 5 to 5.1. However there are a couple of things to consider (e.g. the used database). Please take a look at http://kb.vmware.com/kb/2021193 for details.
vCenter 5.1 supports ESXi 5 hosts, so you can upgrade the hosts at a later time (see VMware Product Interoperability Matrix)
André