What happens if you don't pay your taxes? (see details)

So, I have a bit of money that I made on the side in 2009 at a hospital in town, being a model patient for first-year doctors. I was told I would be hired as a private contractor. When tax time 2010 rolled around, I had forgotten about these jobs I had done in 2009, and filed my taxes with my main day job, because this is the way it has always been for me – filing for just one job with the 1040-EZ. The IRS sent me a notice last November that I owed back taxes from the second job, because the employer had sent in a 1099 on me.

I have let it sit for months now, mostly because other things happen and it gets pushed into a corner, and I got a certified letter the other day from the IRS, again about the back tax money owed. I will, of course, pay them, since it is a nominal amount of money, but what would happen if I let it go…what would their procedure be, and how would this accrue over time? Would I end up in prison for $200?