I'm a bit lost as to your description....
Normally it should go into slumber (sleep) when it is not in use for 15 minutes, however after installing iLife09 (perhaps this is just a coincidence) it switches off after 15 minutes.
This implies that all programs are switched off and I have to restart everything. This is very anoying especially when I am doing some research and all webpages are closed.
This is the bit that is really throwing me! Does your computer shut down, or does the screen go to sleep and the computer is still powered up?
I think the energy settings (as per the above post) are at play here, unless I'm missing something.
Also, I have to ask, and please don't take offence... is iLife 09 from an install disc, or a download of some sort?