Oblivious Turing Machine emulation lower bound
As mentioned above, it is not known in general if there is a faster oblivious simulation.

But interesting lower bounds for this problem are known, under more constrained conditions. For instance, what if you want an oblivious simulation that preserves not only the time $t$ but also the space usage $s$? Beame and Machmouchi have recently proved an interesting time-space tradeoff lower bound for this problem: either the space must increase by a factor of $n^{1-o(1)}$, or the time must increase by a factor of $\Omega(\log n \cdot \log \log n)$.

The paper is here: http://eccc.hpi-web.de/report/2010/104/