I used to be an H1B for 4 years and started school last year in 2005 on F1. I filed 1040 last year for 2005 since I met the 183 day test. Am I supposed to file a 1040 or a 1040 NR for 2006?

If you were F-1 for ALL of 2006, then file Form 1040NR for 2006.