Sony have filed a patent for a system whereby the PS3's Cell Processor could emulate the hardware of a PS2, opening up the older console's back-catalog. The patent describes how the Cell Processor would emulate the Emotion Engine chipset that was included in the first-generation of PS3 hardware but then removed midway through 2007.
The Emotion Engine was used to allow PS3 gamers to use their older, PS2 titles on the new machine. Present on the original 60GB PS3, it was removed when the 80GB version arrived, and was also absent from the entry-level 40GB model.
Sony describe the new system as "an example of a host system based on a cell processor that may be configured to emulate the target system", and there seems little purpose to it beyond backwards-compatibility. One possibility is that Sony are considering digital distribution of their PS2 back-catalog, making classic titles available to all PS3 gamers.