I see that there are already petitions on their way to the President to try her under Federal jurisdiction.....I got this one on Facebook today.
If anyone wants to sign it, I'll give you the link.

Under the separate sovereign doctrine, a defendant may be prosecuted for the same offense by separate sovereigns, such as State and Federal governments. Heath v. Alabama, 474 U.S. 82, 88, 88 L. Ed. 2d 387, 106 S. Ct. 433 (1985)

Florida v. Casey Marie Anthony was state-level, and the FBI's involvement - along with false statements made to the FBI by Ms. Anthony - makes it possible for the Federal Government to prepare a case against Casey Marie Anthony.

The Fifth Amendment regarding "Double Jeopardy" and its exceptions, which include prosecution by separate sovereigns so long as each unit draws their authority from separate sources of power:

http://www.gpoaccess.gov/constitution/pdf2002/023.pdf