  The upper critical field Hc2(T) for MgB2 is analyzed in terms of single and
multi-band Eliashberg models. The relatively high value of Hc2(0) in the range
of 14 to 18 Tesla can be understood, if a strongly coupled subgroup of heavy
quasiparticles is involved in the superconductivity. The sizable coupling to
both low- and high-frequency bosons is essential. This picture is supported by
LDA calculations of Fermi velocity distribution over different sheets of the
Fermi surface with special emphasis on the hole tubes. The possible origin of
anharmonic soft modes is briefly discussed. Similarities and differences with
transition metal borocarbides are figured out.
