Like most users I have had the problem of a sleeping Mac failing to awaken. Recently, upon putting the PowerBook G4 TI to sleep, I have waited until I saw the little pulsing light at the base of the screen before shutting the lid. Since then I have not had the machine fail to awaken. It may be I just haven't been doing this long enough, but so far so good.
I think what may happen is the lid gets shut before the full sleep sequence has completed and it gets interrupted. I don't know that, but I will consider this a solution until it fails.