  We present new lattice results of the K -> pion semileptonic form factors
obtained from simulations with two flavors of dynamical twisted-mass fermions,
using pion masses as light as 260 MeV. Our main result is f+(0) = 0.9560 (84),
which, combined with the latest experimental data for Kl3 decays, leads to
|V_{us}| = 0.2267 (5)_exp (20)_f+(0). Using the PDG(2008) determinations of
|Vud| and |Vub| our result implies for the unitarity relation |Vud|**2 +
|Vus|**2 + |Vub|**2 = 1.0004 (15). For the O(p**6) term of the chiral expansion
of f+(0) we get Df = f+(0) - 1 - f2 = -0.0214 (84).
