there is software called WakeOnMAc - which sends the correct commands over the network to wake a computer from sleep:

http://www.versiontracker.com/dyn/moreinfo/macosx/30394

not all computers support this, and i dont believe it can be done over wireless.

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the energy saver control panel supports timed shut down and wake schedules in the "schedules" options.

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there might be additional 3rd party solutions but i dont know