I wouldn't count on any plug-in hybrids (PHEV;s) in 2008, actually. The major auto manufacturers are still dragging their feet on anything that doesn't get its fuel from a gas pump.
My dad has a Cadillac DeVille and usually can get 26-29mpg on the highway (70mph, AC usually cranked). It's a 1999 version, not expensive these days, it's big, quiet, and comfy. My father-in-law squeezed 30mph from his Buick Park Avenue circa 1999. If you want to buy a car today that gets "significantly" better mileage (say 40mpg...or even 35), you are looking at a pretty small number of options...or just get an older buick or caddie and enjoy the ride (and price) Now some of these diesel or hybrid cars getting over 50mpg, now that is an improvement, no doubt. But why so few choices? I just read Consumer Reports update on some minivans and most were in the mid to upper teens as far as mileage. 16mpg? Heck, my friggin' old truck gets 13.5 in town and 15.5 on the highway! And now, no plug-ins on the horizon. I'm just fed up with the car manufacturers and oil companies! Sorry to go totally OT but as far as I can see, honda or toyota hybrids or maybe a euro diesel car are the only ways to go.