  We present the first mid-infrared Spitzer/Infrared Spectrograph (IRS)
observations of powerful radio galaxies at z>2. These radio galaxies, 4C +23.56
(z=2.48) and 6C J1908+7220 (z=3.53), both show strong mid-infrared continua,
but with 6C J1908+7220 also showing strong PAH emission at rest-frame 6.2 and
7.7um. In 4C+23.56 we see no obvious PAH features above the continuum. The PAH
emission in 6C J1908+7220 is the amongst the most distant observed to date and
implies that there is a large instantaneous star formation rate (SFR). This is
consistent with the strong detection of 6C J1908+7220 at far-IR and sub-mm
wavelengths, indicative of large amounts of cold dust, ~10^9Msun. Powerful
radio galaxies at lower redshifts tend to have weak or undetectable PAH
features and typically have lower far-IR luminosities. In addition, 4C 23.56
shows moderate silicate absorption as seen in less luminous radio galaxies,
indicating tau_{9.7um}=0.3+/-0.05. This feature is shifted out of the observed
wavelength range for 6C J1908+7220. The correlation of strong PAH features with
large amounts of cold dust, despite the presence of a powerful AGN, is in
agreement with other recent results and implies that star formation at high
redshift is, in some cases at least, associated with powerful, obscured AGN.
