Can I use variables on an IPython notebook markup cell?

If you don't mind a code cell that does the job, there is a possibility without adding any extensions.

from IPython.display import Markdown as md

fr=2 #GHz

md("$f_r = %i$ GHz"%(fr))

This will show a markdown cell in a nicely LaTeX formatted output

Currently, this is not possible, however there is a large discussion on this topic here https://github.com/ipython/ipython/pull/2592. The PR is currently closed, but a corresponding issue is opened https://github.com/ipython/ipython/issues/2958 and marked as wishlist.

Update

In the meantime an IPython extension has appeared which allows to render python variables in markdown cells. This extension is part of the IPython notebook extensions and works with IPython 2.x and 3.x. For a detailed description see the wiki page.