How to obtain a dual power supply ±2.5V from a battery for a portable system?
Creating a virtual ground using LDO is possible if you have a 5 V source, as mentioned. here is one such reference:
http://cds.linear.com/docs/en/design-note/dn115f.pdf



In order to have a stable 5 V, i would go for any boost converter. Below is one such example considering 3.7 V input supply available.
http://www.linear.com/product/LT1307


Points:


Since the negative ref. voltage is only needed for ADC, the current requirement will be too low. A resistor divided may also be sufficient instead of dedicated linear 2.5 V regulator for virtual ground.
Also, since 5 V is from a regulator with low noise and small quiescent current, it is suitable for battery powered application.
using second LDO for positive 2.5 V reference, keeps the supplies free form noise (less noise)


Everything assuming a current need of less than 200 mA at 5V.