In the following example code the height of the second row of text increases due to the presence of the inline math formula:
\documentclass{article}
\linespread{0.9}
\begin{document}
Lorem ipsum dolor sit amet, consectetuer adipiscing elit.
Ut purus elit, vestibulum ut, placerat ac, adipiscing vitae,
felis. $A_n^{\{x\}}$ Curabitur dictum gravida mauris. Nam arcu
libero, nonummy eget, consectetuer id, vulputate a, magna.
\end{document}
You can \smash
the inline math formula:
\documentclass{article}
\begin{document}
% Default layout/spacing
Lorem ipsum dolor sit amet, consectetuer adipiscing elit.
Ut purus elit, vestibulum ut, placerat ac, adipiscing vitae,
felis. $A_n^{\{x\}}$ Curabitur dictum gravida mauris. Nam arcu
libero, nonummy eget, consectetuer id, vulputate a, magna.
% Smashed inline math
Lorem ipsum dolor sit amet, consectetuer adipiscing elit.
Ut purus elit, vestibulum ut, placerat ac, adipiscing vitae,
felis. \smash{$A_n^{\{x\}}$} Curabitur dictum gravida mauris. Nam arcu
libero, nonummy eget, consectetuer id, vulputate a, magna.
\end{document}
However, it's obvious that tall [deep] inline expressions may butt into descenders [ascenders] of the line above [below].