Have a read here:
a good summary of the 802.3af standard
http://en.wikipedia.org/wiki/802.3af
"Two modes, A and B, are available. In mode A, pins 1-2 (pair #2 in T568B wiring) form one side of the 48 volt DC supply, and pins 3-6 (pair #3 in T568B) provide the 48 V return. These are the same two pairs used for data transmission in 10Base-T and 100BASE-TX, allowing the provision of both power and data over only two pairs in such networks.
In mode B, pins 4-5 (pair #1 in both T568A and T568B) form one side of the DC supply and pins 7-8 (pair 4 in T568A and T568B) provide the return; these are the "spare" pairs in 10BASE-T and 100BASE-TX. Mode B, therefore, requires a 4-pair cable.
The power sourcing equipment (PSE) decides whether power mode A or B is to be used, not the powered device (PD).
The PSE can implement mode A or B or both (but must not supply power in both modes at the same time)."