Proving OLS unbiasedness without conditional zero error expectation? The OLS estimate $b$ is equal to $(X^TX)^{-1}X^Ty$ for the linear regression model. If we assume that $E(\epsilon|X)=0$ then it is easy to prove simply by taking the conditional expectation, of $b$ substituting in the expression for $y$ and simplifying.
But how do we prove it if we only know that $E(X^T\epsilon)=0$? $E(\epsilon|X)=0$ implies $E(X^T\epsilon)=0$, but not the other way around.
EDIT: Can I just get an answer, just to be sure, that even if the $u_i$'s are i.i.d., that $E(x_iu_i)=0$ does not imply unbiasedness? 
So just to be absolutely clear: If $u_i$ is i.i.d., and we know that $E(x_iu_i)=0$ holds, but we don't know whether $E(u_i|x_i)=0$ holds, then OLS may be biased?
 A: For this question we can make use of a simple decomposition of the OLS estimator:
$$\begin{equation} \begin{aligned}
\hat{\boldsymbol{\beta}} = (\mathbf{X}^\text{T} \mathbf{X})^{-1} \mathbf{X}^\text{T} \mathbf{Y} 
&= (\mathbf{X}^\text{T} \mathbf{X})^{-1} \mathbf{X}^\text{T} (\mathbf{X} \boldsymbol{\beta} + \mathbf{\epsilon}) \\[6pt]
&= \boldsymbol{\beta} + (\mathbf{X}^\text{T} \mathbf{X})^{-1} \mathbf{X}^\text{T} \mathbf{\epsilon}. \\[6pt]
\end{aligned} \end{equation}$$
This useful decomposition follows directly from the form of the OLS estimator and the underlying regression equation, so it is not dependent on any assumptions about the behaviour of the error terms.  From this decomposition, the conditional bias (taking the regressors as fixed) is:
$$\text{Bias}(\hat{\boldsymbol{\beta}}|\mathbf{x}) = \mathbb{E}(\hat{\boldsymbol{\beta}} | \mathbf{x}) - \boldsymbol{\beta} = (\mathbf{x}^\text{T} \mathbf{x})^{-1} \mathbf{x}^\text{T} \mathbb{E}(\mathbf{\epsilon}| \mathbf{x}).$$
The unconditional (marginal) bias (taking the regressors as random variables) is:
$$\text{Bias}(\hat{\boldsymbol{\beta}}) = \mathbb{E}(\hat{\boldsymbol{\beta}}) - \boldsymbol{\beta} = \mathbb{E}((\mathbf{X}^\text{T} \mathbf{X})^{-1} \mathbf{X}^\text{T} \mathbf{\epsilon}).$$
In both cases, the condition $\mathbb{E}(\mathbf{\epsilon}| \mathbf{x}) = \mathbf{0}$ is sufficient for unbiasedness, but in the latter case, the weaker condition $\mathbb{E}((\mathbf{X}^\text{T} \mathbf{X})^{-1} \mathbf{X}^\text{T} \mathbf{\epsilon}) = \mathbf{0}$ is sufficient.  The condition $\mathbb{E}( \mathbf{X}^\text{T} \mathbf{\epsilon}) = \mathbf{0}$  is not sufficient for unbiasedness in either case.
A: You can't, because the statement is not true under the weaker assumption.
Consider for example the autoregressive model
\begin{equation*}
y_{t}=\beta y_{t-1}+\epsilon _{t},
\end{equation*}
in which the strict exogeneity $E(\epsilon|X)$ is violated even under the assumption $E(\epsilon_{t}y_{t-1})=0$: 
we have that
\begin{equation*}
E(\epsilon_ty_{t})=E(\epsilon_t(\beta y_{t-1}+\epsilon _{t}))=E(\epsilon_{t}^{2})\neq 0.
\end{equation*}
But, as $y_{t+1}=\beta y_{t}+\epsilon_{t+1}$, $y_t$ is also a regressor for $y_{t+1}$ and hence, it is impossible in this model that the error term is also uncorrelated with future regressors.
Now, it is also well-known that OLS is biased for the coefficient of an AR(1)-model, see Why is OLS estimator of AR(1) coefficient biased?
