There is a quick FAQ on Oracle VM, explaining the basics about Oracle new virtualization technology. For instance, basic OracleVM is free as anyone can download the binaries. Support is available on a per-socket basis, not per-core. There are also ready-made templates with Oracle Linux (OEL) 4 update 5 for both 32 bit and 64 bit VMs. You will manage VMs with Oracle VM Manager.
Oracle supports their own database 10g and 11g inside OracleVM, as well as OEM, Siebel, Peoplesoft and E-business Suite. Support for Oracle RAC clusters will be available somewhere in 2008. Testing is in progress, but timing issues in RAC are very strict, so it must be thoroughly tested.
The requirements for OracleVM as similar to Xen's requirements: a new/recent CPU with VTX virtualization extensions is required for running VMs with Windows. If you only run Linux, you can use older hardware as well (and a kernel supporting paravirtualization).