I wouldn't worry about a cover letter for a job fair - the cover letter is used to introduce you and point out parts of your resume that show how well you match up to the published requirements of the job, typically if your submission was/is unsolicited...like if you were trying to get a corporate flight job when they haven't advertised and opening. At a job fair - you are going to do that in person, and they will hold the resume with the others they talk to. It's common for them to take notes over/on it while you talk.

If you want to leave an impression, make up business cards with your contact info and on the back state the position you are looking and quick summary of your experience. Then hand them to everyone you talk to.

Common theme from Spongebob's threads: Just make sure you do some research on each company (google is wonderful, as is yahoo finance) you plan to talk to so you can hold an intelligent and meaningful conversation.

HTH
Spongebob