Running suspicious software in a virtual machine is a basic precaution to determine if the software contains malicious code.
However, it is generally easy to detect whether software runs inside a VM. RootForce lists ways to detect a VirtualBox VM from the guest OS, including obvious naming issues and more subtle methods.
A proof-of-concept 'malware' application, Al-Khaser, demonstrates this and can be used to verify anti-malware systems, such as when debugging malware in a VM.
Running suspicious software in a virtual machine seems like a basic precaution.
Author's summary: Virtual machines can be detected by malware, compromising security tests.