I've been known to say "quit and get a new job" myself a fair number of times. In my own case, it's born out of the evidence that I see every day. When I say it, I'm not talking about it being easy to find a job, not necessarily your dream job.
So, what's my evidence? I do fulltime contract work in Minneapolis. In 2007, I had quite a few short projects and made 5 transitions before August. The longest it took to find one of those gigs was 3 days. In one case, I got the first call that the client was interested at 2:00 and had a signed contract 3 hours later without having even met anyone there in person. None of the gigs was any further than a 25 minute drive.
I generally leave my resume active on Monster and Dice year-round. Until I changed my answering machine message to make it clear that I only do contract work and am booked through the end of 2008, I averaged 12 calls about new jobs/gigs per week.
I won't pretend that those calls represent the kind of dream job that lots of developers want. Many of them don't pay that well. Many are high pressure for little recognition/respect. Heck, one of my gigs in 2007 was an disaster that I walked away from after 2.5 weeks. But, they're jobs in software development, near my home (which isn't in Silicon Valley or New York, remember).
If faced with an unexpected loss of gig/job in the near future, I am dead certain that I could find a job doing software development here in the Minneapolis area, paying better than the state median household income long before a financial crunch would occur.
Incidentally, just being confident that you could get another job easily can give you the push to actually make your existing situation better. Lots of times, people hold back on taking risks at work because they're afraid of losing their job. Those risks are often what it takes to really instigate change in an environment.
If nothing else, it dramatically changes the kinds of conversations where a project manager is pushing you to work until 11:00pm on Saturday night "or else".
Just ask anyone who teaches negotiation for things like buying a car. The single biggest thing you can do to make your position stronger is being 100% willing to walk away from the sale. The same holds true in a job. When a boss is screaming that you had better get this release working by the end of the day, there's always an implied threat of firing. A response of "go ahead and fire me. If you think that firing me will get this done faster than having me stay and help you, have at it" brings a little civility to the conversation.
All of that said, the economy always plays a part and you've got to stay plugged in to what's going on. I recommend going on interviews regularly even if you're not actually looking for a job. It gives you a better feel and keeps your interviewing skills sharp. That can make a HUGE difference in getting gigs/jobs easily. I know that if I get an interview from a resume submission, I've got an 85% chance of getting an offer because that's what my track record has proven (and not based on 1-2 situations).