  Recent second-order perturbation computations have provided an accurate
prediction for the primordial gravitational potential $\Phi(x)$ in scenarios in
which cosmological perturbations are generated either during or after
inflation. This enables us to make realistic predictions for a non-Gaussian
part of $\Phi(x)$, which is generically written in momentum space as a double
convolution of its Gaussian part with a suitable kernel, f_NL(k1,k2). This
kernel defines the amplitude and angular structure of the non-Gaussian signals
and originates from the evolution of second-order perturbations after the
generation of the curvature perturbation. We derive a generic formula for the
CMB angular bispectrum with arbitrary f_NL(k1,k2) and examine the detectability
of the primordial non-Gaussian signals from various scenarios such as
single-field inflation, inhomogeneous reheating, and curvaton scenarios. Our
results show that in the standard slow-roll inflation scenario the signal
actually comes from the momentum-dependent part of f_NL(k1,k2), and thus it is
important to include the momentum dependence in the data analysis. In the other
scenarios the primordial non-Gaussianity is comparable to or larger than these
post-inflationary effects. We find that WMAP cannot detect non-Gaussian signals
generated by these models. Numerical calculations for l>500 are still
computationally expensive, and we are not yet able to extend our calculations
to Planck's angular resolution; however, there is an encouraging trend which
shows that Planck may be able to detect these non-Gaussian signals.
