It would be extremely difficult -- if not impossible -- for someone to truly steal your identity just by gaining access to the information on your computer. If you've scanned in images of your driver's license, store credit card numbers, bank account numbers, or other information that really shouldn't be on there anyway, then identity theft is a minute possibility, but still a far-fetched idea.
FileVault will protect everything contained in your home folder, and nothing else. For a system like mine with multiple drives, and data frequently stored outside the home folder, FileVault would do nothing but protect my bookmarks and application settings... for a laptop, though, with a single partition, it may be a viable solution, but I think you need to ask yourself, "Is there information sensitive enough to warrant encryption on my drive?" Term papers, emails to friends, little bits of code you've written -- these things don't need 128-bit AES secure encryption. So what if someone gets their hands on them? If you happened to steal someone's laptop, would you think you hit the motherload when you find last semester's papers or some emails between he and his girlfriend? Naah... people don't want your personal stuff. It's useless to them. Still, if you feel the need for FileVault, I would make sure your drive is in pristine condition (permissions repaired, no disk errors) and I would "clean house" on the home folder, just so FileVault keeps the encrypted disk image size to a minimum.
Instead of FileVault, I would suggest a good, hard-to-guess password, and perhaps setting the open firmware password. That's good enough to keep 99% of the thieves out of your stuff, and the remaining 1% probably wouldn't be interested in your personal stuff anyway. Their only option would be to reformat the drive, granted they knew enough about Macs to open it up and remove/add RAM to get around the open firmware password.
I think you need to worry more about the actual laptop itself rather than what's stored on it. Keep a good backup, as you said, just in case of a theft, but other than that, just keep your eye on your 'book and if you see anyone eyeing it suspiciously, prompty give them a swift kick to the head!
