The fine folks over at PS3 Fanboy have managed to confirm
that PS3 owners will indeed be able to download content from the PlayStation Network while playing single player gamers. The downloads will be queued when playing online games which is understandable. So to summarize, the background downloading for the PS3 will work exactly like it does on the Xbox 360.
The patch will be available in the 1.6 firmware patch which will also include the Folding@Home client.