Hi Eric,
Text and images can coexist in most word processing apps such as Pages, TextEdit, Word as well as with HTML. As Kyn pointed out, the text simply flows around the image. However in layout and presentation apps text and images are assigned as separate objects. As such it is not possible to imbed an image in a text object. This is also the way it works in Powerpoint, Photoshop and Illustrator. So I doubt that Keynote will ever have this ability.
One solution is to type the entire paragraph or sentence in LaTeXit and render the equation along with the rest of the text. That way the text and the equation will included in the PDF image. You just need to place "$" signs around the equation and click the "Text" tab in the main window of LaTeXit. For example:
Here we will use the Schrodinger equation, $-\frac{\hbar^{2}}{2m}\nabla^{2}\Psi+V\Psi=E\Psi$, in a sentence.
Hope that helps,
Dave