From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156724-112796
Question: My lawn looks fairly healthy but is very hard under foot.
Would aerating help?
Answer: If your soil is composed of compacted clay, it will feel quite
hard. Another factor is the type of grass; some forms a denser mat
than others. If compaction is the problem, aerating will definitely
help loosen the soil and allow water and air to reach the grass roots,
which will improve growth. You can go a step further and sprinkle
screened compost on your lawn with a fertilizer spreader after you
aerate, which will encourage the presence of earthworms and other
organisms that help aerate and loosen the soil. Avoid using chemical
weed killers because they can harm soil life. Mow high to reduce weed
growth, and leave grass clippings on the lawn to contribute organic
matter and improve the health of your lawn.