  We present a measurement of the angular power spectrum of the cosmic
microwave background (CMB) radiation observed at 148 GHz. The measurement uses
maps with 1.4' angular resolution made with data from the Atacama Cosmology
Telescope (ACT). The observations cover 228 square degrees of the southern sky,
in a 4.2-degree-wide strip centered on declination 53 degrees South. The CMB at
arcminute angular scales is particularly sensitive to the Silk damping scale,
to the Sunyaev-Zel'dovich (SZ) effect from galaxy clusters, and to emission by
radio sources and dusty galaxies. After masking the 108 brightest point sources
in our maps, we estimate the power spectrum between 600 < \ell < 8000 using the
adaptive multi-taper method to minimize spectral leakage and maximize use of
the full data set. Our absolute calibration is based on observations of Uranus.
To verify the calibration and test the fidelity of our map at large angular
scales, we cross-correlate the ACT map to the WMAP map and recover the WMAP
power spectrum from 250 < ell < 1150. The power beyond the Silk damping tail of
the CMB is consistent with models of the emission from point sources. We
quantify the contribution of SZ clusters to the power spectrum by fitting to a
model normalized at sigma8 = 0.8. We constrain the model's amplitude ASZ < 1.63
(95% CL). If interpreted as a measurement of sigma8, this implies sigma8^SZ <
0.86 (95% CL) given our SZ model. A fit of ACT and WMAP five-year data jointly
to a 6-parameter LCDM model plus terms for point sources and the SZ effect is
consistent with these results.
