  We monitored the flaring blazar 3C 454.3 during 2005 June-July with the
Spitzer Infrared Spectrograph (IRS: 15 epochs), Infrared Array Camera (IRAC: 12
epochs) and Multiband Imaging Photometer (MIPS: 2 epochs). We also made Spitzer
IRS, IRAC, and MIPS observations from 2006 December-2007 January when the
source was in a low state, the latter simultaneous with a single Chandra X-ray
observation. In addition, we present optical and sub-mm monitoring data. The
2005-2007 period saw 3 major outbursts. We present evidence that the
radio-optical SED actually consists of two variable synchrotron peaks, the
primary at IR and the secondary at sub-mm wavelengths. The lag between the
optical and sub-mm outbursts may indicate that these two peaks arise from two
distinct regions along the jet separated by a distance of 0.07-5 pc. The flux
at 5-35 microns varied by a factor of 40 and the IR peak varied in frequency
from <1E13 Hz to 4E13 Hz between the highest and lowest states in 2005 and
2006, respectively. Variability was well correlated across the mid-IR band,
with no measurable lag. Flares that doubled in flux occurred on a time scale of
3 days. The IR SED peak moved to higher frequency as a flare brightened, then
returned to lower frequency as it decayed. The fractional variability amplitude
increased with frequency, which we attribute to decreasing synchrotron-self
absorption optical depth. Mid-IR flares may signal the re-energization of a
shock that runs into inhomogeneities along the pre-existing jet or in the
external medium. The synchrotron peak frequencies during each major outburst
may depend upon both the distance from the jet apex and the physical conditions
in the shocks. Variation of the Doppler parameter along a curved or helical jet
is another possibility. Frequency variability of the IR synchrotron peak may
have important consequences for the interpretation of the blazar sequence, and
the presence of a secondary peak may give insight into jet structure.
