Can an employer replace you because they think you are leaving before you have formally resigned?

In April 2010 I spoke with my direct supervisor about what my options would be if I started an internship in August 2010. Around May 2010 I was informed that they wouldn't be able to work with me. I informed my direct supervisor that I would let her know what I decided to do once it came to that time. In July 2010 my direct supervisor asked if I would be able to stay if they could do a special schedule for me. I informed her to let me know what the schedule would be and I would decide. On July 22, 2010, my direct supervisor called me into her office with her supervisor. Her supervisor asked me when my last day would be and I informed her that I wasn't for sure at that time, but I would let her know 2 weeks prior to last day as the policy stated. She then asked could I give her something on paper that day. I told her I couldn't, but that she would get a 2 weeks notice. She then told me that she needed to know something so she could tell the corporate because they have already replaced me in the budget. I informed her again that I would give a 2 weeks notice. Prior to that day, i had not spoke with my supervisor's supervisor about the possibulity of me leaving. As of 8/3/10, I still have not turned in a formal resignation. IS there anything I can do about this? Thanks