Found a good one, as it turned out. Rain Design in San Francisco (www.raindesigninc.com) makes a terrific lap stand called -- inevitably -- "iLap." It comes in sizes for 12, 14, 15, and 17 inch notebooks, as well as 15 and 17 inch widescreens. Prices range from $50-$70 -- it's not inexpensive, but sometimes you really do get what you pay for.
iLap is made of substantial aluminum stock, a sharp contrast from similar products I've tested that bend and crease at the slightest movement. At the back, iLap has a padded and hinged u-shaped arm at the back, which props your laptop up at an ergonomically correct angle. Used in your lap, the padded arm sits comfortably around your knee.
At the front, there's a thick, rounded cushion that does triple duty: cushions your lap, positions the keyboard properly with respect to your hands, and acts as a wrist rest. This will work well for most users, although if like me you are, ahem, waistline-challenged it may push your laptop a little too far from your body.
The cushion is attached with Velcro and can be detached to let you use the iLap on your desk. There are cheaper solutions for the desktop, though. To justify an investment in an iLap, your primary need should be as a lap desk for a laptop. Think of desk use as an added bonus.
As for me, I'm heading out to my patio and taking my iLap and PowerBook with me.
