I'm a big guy (six feet five, 270 pounds) who's interested in buying a hybrid bike for some urban biking and trail riding (nothing too extreme or competitive). So I need a bike that fits but doesn't cost a fortune. Also, can you explain mountain- and hybrid-bike sizing conventions, especially as there doesn't seem to be a standard way of measuring hybrid bikes?
The issue of sizing bicycles is so confusing that a lot of makers have abandoned using inches or centimeters and now just size them from small to extra large; you'd clearly fall into the latter category. In general, frame sizing is based on the height of the frame, minus the wheels. Hence, a 21-inch road bike fits my 30-inch inseam. Mountain bikes have a "shorter" frame so you can dismount more easily, making up for that with a taller seatpost and handlebars. They're usually about four inches shorter than an equivalent road bike; sure enough, my Marin mountain bike is a 17-incher. But still, every maker has its own little sizing tricks, so it's difficult to say "buy an X" and solve your problems. You live in Seattle, though, so I should think you could find a bike shop where you can try several bikes at once and find one that works.
That aside, I'm inclined to say what you want is a true mountain bike, which doesn't necessarily mean spending a fortune. I don't want to suggest that hybrids are delicate, because they're not. But your 270 pounds will put plenty strain on the wheels, suspension, and so on. Most good hybrids, such as the Cannondale Adventure 500, sell in the $500 range. A decent hardtail mountain bike will run a little higher, but not by a mile. Take a look at something like a Giant Iguana ($500; www.giant-bicycles.com) or Raleigh's M600 ($700; www.raleighusa.com). Both are nice bikes with aluminum frames, good front shocks, and decent components.
For smoother riding on pavement and good trails, have the dealer swap out the stock knobby tires for something like an Avocet Cross, which has a recessed tread that runs smoothly on hardpack, but also grips pretty well in soft dirt or sand.