9 páginas.-- Comunicación presentada a la Kaon International Conference (KAON'07) celebrada del 21 al 25 de MAyo de 2007 en Frascati (Italia).A very precise determination of V_us can be obtained from the semi-inclusive hadronic decay width of the tau lepton into final states with strangeness. The ratio of the Cabibbo-suppressed and Cabibbo-allowed tau decay widths directly measures (V_us/V_ud)^2, up to very small SU(3)-breaking corrections which can be theoretically estimated with the needed accuracy. Together with previous LEP and CLEO data, the recent measurements by Babar and Belle of some Cabibbo-suppressed tau decays imply V_us= 0.2165 +- 0.0026_exp +- 0.0005_th, which is already competitive with the standard extraction from K_l3 decays. The uncertainty is largely dominated by experimental errors and should be easily reduced with the high statistics of the B factories, providing the most accurate determination of this parameter. A 1% experimental precision on the Cabibbo-suppressed tau decay width would translate into a 0.6% uncertainty on V_us.This work has been supported by the European Commission MRTN FLAVIAnet [MRTN-CT- 2006-035482], theMEC (Spain) and FEDER (EC) [FPA2005-02211 (M.J.), FPA2004-00996 (A.P.) and FPA2006-05294 (J.P.)], the Deutsche Forschungsgemeinschaft (F.S.), the Junta de Andalucía [P05-FQM-101 (J.P.), P05-FQM-437 (E.G. and J.P.) and Sabbatical Grant PR2006-0369 (J.P.)] and the Generalitat Valenciana [GVACOMP2007-156 (A.P.)].Peer reviewed