Interpret found that, during 2007, casual games reached over 145 million people aged 12-65. Of that figure over 71 million play casual games for one or more hours per week.
There's a new media research firm on the block, and they've done their homework. (Or, more accurately, they've interrogated 8000 people said to be representative of a cross sections of American society. That's interesting because it means the figures above are only those people in the US playing casual games.) The biggest gaming market in the world is only getting bigger, according to this report.
Perhaps worth remembering that PopCap's site alone handles nearly 6 million unique visitors each month. Do you play block sorting games on a regular basis, readers?