At 10/100Mbps, they aren't used for anything. In fact, I've made special cables that use those pairs for other data, just like you are suggesting. Note that if you do too much with them though, you risk electrical interference with the data carried over the pairs, so you need to be careful. But I've done things like split them out so that I can carry two 10/100 Ethernet runs, or carried voice over the other pairs (just make sure you don't wire up the pairs into the same jack that the ethernet is on, or you'll risk frying your NIC).
EDIT: Answering question in the comments: well, if you are running ethernet over the second two pairs, sure, it should be fine. If you are running something else, then probably not. If you are lucky things will just fail. If you are unlucky, you'll fry your switch or whatever other device you've got connected to the hacked pairs.
EDIT2: Evan prompted me to double check wht I was saying here, and I found this interesting section in the wikipedia article on Ethernet over twisted pair which also points out the bit about hubs short-circuiting the unused pair:
10BASE-T and 100BASE-TX only require
two pairs to operate, located on pins
1 plus 2 and pins 3 plus 6. Since
10BASE-T and 100BASE-TX need only two
pairs and Category 5 cable has four
pairs, it is possible, but not
standards compliant, to run two
network connections (or a network
connection and two phone lines) over a
Category 5 cable by using the normally
unused pairs (pins 4–5, 7–8) in 10-
and 100-Mbit/s configurations. In
practice, great care must be taken to
separate these pairs as most
10/100-Mbit/s hubs, switches and PCs
internally hardwire pins 4–5 together
and pins 7–8 together, thereby
creating a short-circuit across each
"unused" pair. Moreover, 1000BASE-T
requires all four pairs to operate,
pins 1 and 2, 3 and 6 — as well as 4
and 5, 7 and 8.