Yes. I have posted here previously on how we have actually seen nominal interest rates below zero, including recently for both the actual federal funds rate and certain Treasury bill rates. During August to November of 2003, the repo market rates regularly went negative, this being the market the Fed normally uses for controlling the federal funds rate. And Japan had negative rates off and on in the late 1990s. Thus, if actual rates were to go negative and stay, the Fed could push the target rates below zero. This situation might well arise if the economic crisis worsens severely, and we fall into deflation. This would open up a new tool for the Fed, overcoming the limits of the liquidity trap.
The main theoretical argument for why interest rates cannot be negative, or not over a sustained period, as it is now clear that they can be so for at least short periods of time, has been the argument of cash as an alternative. That there was a lot of cash around in the 1930s may well have been why we never saw negative interest rates during that period of deflation and extreme economic decline. However, now cash is a tiny fraction of the money supply and of wealth more generally. It is not a meaningful alternative to government securities on a large scale for serious wealth holders, and such alternatives as checking accounts or CDs are all ultimately backed by government securities anyway, if the FDIC were to go under in a general further wave of bank collapses. Under such circumstances, the negative interest rate tool may be the only way out, especially if this follows a failed fiscal expansion.