Consolidation of server is a very hot topic as server rooms fill up and cooling becomes ever more expensive. Virtualization is the current darling in this space and indeed it is fantastic for many application workloads. However it is not the only solution to the problem and this is particularly true for SQL Server. There are several tools available form Microsoft as well as para virtualization solutions like HP’s Polyserve. Looking at the Microsoft solutions in a little bit more detail:
One thing to note on all of these approaches this is network performance. If you had 2 servers before and you only have 1 now then unless you have multiple network card in the new box the two instances will have contend for the network card.
There is a white paper on SQL Consolidation here
Finally the licensing of all of these options can affect your decision. Enterprise edition makes this quite simple as you license the physical machine per CPU, so you can have as many SQL Server instances, SQL Server virtual machines (any vendor not just Hyper-V) as you can cram on there.