No, what I mean is an LDO usually works by modulating the P-FET pass transistor resistance so that the output voltage stays at a fixed value. That P-FET has a small resistance even when fully on, and this minimum resistance is what actually determines the dropout voltage which itself is therefore linearly dependent on current. If the input voltage is above this dropout voltage then the output is fully regulated because the P-FET's minimum resistance is less than that needed to achieve the desired value. However if you provide the regulator with an input voltage below that dropout voltage then the P-FET is either partially or totally hard on, which is usually a few ohms.
If you look at the regulator spec there's a minimum voltage specified which is 2.7V, this is actually the voltage at which the internal logic is fully functional and the P-FET can be driven properly internally.
In other words, the regulator should allow the voltage through but with a small resistance in series if the input is below the dropout. The output voltage might then be modulated itself by the current it's taking, but that might only be a few hundred millivolts or so, so if the minimum voltage you need to keep things working can cope with those dips then it's entirely possible to use voltages below 3.3V. In theory if the output was taking very little current this would work with an input voltage right the way down to 2.7V.
Mark.
Edit: Here's an example of an LDO doing this, see Figure 6. It's curious why the MCP1703 is not specified like this and has no equivalent graph, but this is generally how LDOs work. That's why I thought it might possibly be a bit quirky, but I'm willing to lay odds on it actually doing this:
http://www.st.com/resource/en/datasheet/stlq015.pdfAlso this explains why the dropout voltage changes with input voltage. The higher the input voltage the harder on the P-FET is driven (larger Vgs), this reduces the minimum on resistance which reduces the dropout voltage. It all fits ;-)
BTW if this is correct then it's OK to run a Moteino from 2 lithium AA cells into Vin, or 3 alkaline AA cells into Vin with a termination voltage of 1V per cell at full TX power.