There are 2 types of anesthesia programs: categorical and advanced. Categorical include all 4 years. Advanced only includes from CA-1 to CA-3. You have to do a prelim year. When you rank an advanced position, a second rank list opens for you to rank your prelim years. If you are applying now, you would be applying for a 2007 PGY-1 surgery spot and a 2008 PGY-2 (CA-1) anesthsia spot. So you've got several options
1. Apply for prelim surg and anes. If you like surg, drop out of anes and stay with surg, if you're able to switch from prelim surg to categorical surg. If you hate surg, then you suffer for your prelim year and then start anes.
2. Apply for categorical surg. If you like it, great. If you hate it, reapply for anes in Sept 2007 (2009 CA-1). You'll have to take a year off. However, there are quite a few programs with spots for people to come into outside the match, e.g., surgery residents who saw the light and switched to anesthesia.
3. Apply for prelim surg only. If you like it, you'll have to find a program that will accept you for the remaining 4 years. If you don't and want to do anesthesia, you'll have to reapply, and it will be like #2 above.