  We study the QCD corrections at next-to-next-to-leading order (NNLO) to the
cross section for the hadronic pair-production of top quarks. We present new
results in the high-energy limit using the well-known framework of
k_t-factorization. We combine these findings with the known threshold
corrections and present improved approximate NNLO results over the full
kinematic range. This approach is employed to quantify the residual theoretical
uncertainty of the approximate NNLO results which amounts to about 4% for the
Tevatron and 5% for the LHC cross-section predictions. Our analytic results in
the high-energy limit will provide an important check on future computations of
the complete NNLO cross sections.
