Finally found a reference to an old (2005) trade show demonstration .. where the cell processor decoded 48 SDTV MPEG-2 streams simultaneously! No I don't think this was done on a PS3 .. but it shows the power of the cell processor. HDTV is roughly 4 SDTV streams ... so you could infer 12 HDTV streams (maybe 10 to be safe) ... now thats PIP!
From the article:
"Toshiba demonstrated that its Cell microprocessor, jointly developed with the Sony Group and IBM Corp., can simultaneously decode 48 SDTV format MPEG-2 streams. At the COOL Chips VIII event held in Yokohama from April 20 to 22, 2005, the company showed a film demonstrating the decoding process"
Here is the article link: http://techon.nikkeibp.co.jp/english/NEWS_EN/20050425/104149/