  We present the first results for the Kl3 form factor from simulations with
2+1 flavours of dynamical domain wall quarks. Combining our result, namely
f_+(0)=0.964(5), with the latest experimental results for Kl3 decays leads to
|V_{us}|=0.2249(14), reducing the uncertaintity in this important parameter.
For the O(p^6) term in the chiral expansion we obtain \Delta f=-0.013(5).
