Unfortunately - OS X does not include the familiar wake/sleep Energy features of previous system incarnations - and as such there really isn't a method of "starting up" a computer at a specified time.
Now that being said - I have actually managed a workable solution - but it requires a 2nd computer of some sort. This works for me because I have a PC I pulled out a of a dumpster and installed Linux on it to manage my internal LAN at the house. This machine stays on all the time. So what I did was download an install a little unix utility called "wakeonlan". I compiled it and installed it on the Linux box. Wakeonlan sends a special signal across your network to specific sleeping (not shutdown) computers and will wake them up. I simply created a cron job on the Linxu box that fires off the wakeonlan utility everyday at a certain time - and voila - all teh computers in the house are awoken at a specified time.
Granted this doesn't really help you if you only have a single computer - but I thought I would throw it out there anyway.
FYI - wakeonlan will compile and run fine on OSX also.