  We report a preliminary measurement of $\cos2\beta$ in $B^0\to D^{(*)0}h^0$
decays with a time-dependent Dalitz plot analysis of $D^0\to K_S\pi^+\pi^-$,
where $h^0$ is a light neutral meson such as $\pi^0$, $\eta$, $\eta'$ or
$\omega$. The strong phase variation on the Dalitz plot allows the access to
the angle $\beta$ with only a two-fold ambiguity ($\beta+\pi$). Using
$311\times 10^{6}$ $B\bar{B}$ pairs collected at the Babar detector, we obtain
$\cos2\beta = 0.54 \pm 0.54 \pm 0.08 \pm 0.18$, $\sin2\beta = 0.45 \pm 0.36 \pm
0.05 \pm 0.07$, and $|\lambda| = 0.975^{+0.093}_{-0.085} \pm 0.012 \pm 0.002 $,
where the first errors are statistical, the second are experimental systematic
uncertainties, and the third are the signal Dalitz model uncertainties. This
measurement favors the solution of $\beta= 22^\circ$ over $68^\circ$ at an 87%
confidence level.
