What choices do I have for backpacks that are hard to steal from?

http://www.deuterusa.com/ make pretty good ones. Mine is designed to carry (and hide the fact I am carrying) a lap-top in a secret padded compartment in the back.

I have the Crumpler Seven Million Dollar Home for my camera kit and I can recommend their products. Especially for security. Once closed, there's no way that someone could get into it unnoticed. I'm not familiar with their backpacks, but if they follow the same principles, there are several levels of closure ending with massive amounts of Velcro, that on opening could wake a cathedral of entombed kings.

The Lowepro http://products.lowepro.com/product/Flipside-400-AW,2116,14.htm (also a camera bag) has a back entry compartment to protect valuables (in this case, your camera). All of the equipment divider sections in my Lowepro are removable and customizable, so you could take them out or reconfigure them for your own use as a regular backpack.

A rain cover, which can be purchased separately, does a decent job of keeping hands out of zippers. I'm a fan of Eagle Creek, and am currently testing their new http://eaglecreek.com/bags_luggage/backpacks/Thrive-65L-10066/ pack. It has a built-in rain cover, and a secret storage compartment accessible only from the back. It's big enough to fit a few paper back books, and even has a little headphone port if you choose to put your iPod in there. It also has a http://www.grabup.com/uploads/5ae67cec05139ef00da49f9fa034fbda.png where all zippers zip to for locking.

Can't comment directly on the Crumpler backpacks in question, but I own about four of their shoulder / courier bags and they're very well-made, with strong materials, zips etc. Unlike the backpacks with the hidden zips, the bags I have use prodigious amounts of Velcro for their openings, so unless you're completely deaf it probably doesn't matter which Crumpler you get. Great for preventing surreptitious theft; terrible for getting your camera out during a wedding.

A low cost option is a kayaking bag AKA riverbag, they roll multiple times and then snap shut (they also trap air inside and float!). For a given size they are often the best waterproof bag value. I got a 70 liter model with backpack attachments for about 60 bucks, and it wasn't on sale. It works great for carrying a big old winco shopping trip full of groceries on my back while I bicycle. There are inconvenience issues, since it is a toploader only, but if you close it up completely there are four snaps that need to be undone, plus an unroll, before you can reach in and grab stuff.

I once had some stuff stolen not because they opened the bag while i was wearing it but literally cut it open. Make sure the bag and strap material is such that will make it hard to cut with a sharp blade.

Thanks for all of your suggestions. This was my first question on this site, and while I've seen how useful Ask Metafilter is, I didn't know I would get so many good recommendations and ideas.

I wasn't going to mark any answer as best answer, because every single one was helpful. However, Doctor Suarez's recommendation about PacSafe was exactly what I was looking for, so I've selected it as the best answer.