My Mac has insomnia

Any chance it is something like a cat jumping on the keyboard?

What kind of usb peripherals do you have plugged in. Peripherals that doesn't have their own power source (Like a plug in adaptor), can keep your machine from sleep. I have both a scanner and a card reader with no independent power source plugged into a usb hub. If I unplug then my machine sleeps just fine. When they are plugged in, no sleep to be had.

This is for a Mac that won't sleep.
First, read things in Apple doc.#25801 on sleep settings.
Here is Apple doc.#303698 with their tips.
Mine follow;
USB devices and hubs often cause Macs to not sleep or wake quickly from sleep (notably HP printers and scanners, HP all in ones, and Cannon scanners). Disconnect all USB except for keyboard and mouse and try. If it sleeps, add one back at a time testing after each one to find the culprit.

If you've set your Mac to receive faxes, it may not be able to sleep. It might also have a damaged preferences file.

If you use your Mac for faxing, open System Preferences. click Print & Fax, click the Faxing tab, and uncheck the Receive Faxes On This Computer box.

If that doesn't work or you've never faxed on your Mac, go to Finder/Hard Drive /Library/Preferences/SystemConfiguration and delete the com.apple.PowerManagement.plist and com..apple.AutoWake.plist files (if your Mac asks, type your admin. password).

If the latter file isn't present in the folder, go to user name(probably your name)/Library/Preferences, delete the com.apple.systemuiserver.plist file, and reboot your Mac.

Hope this helps!
P.S. One member here reported this (Michael Gleason);
"I have had this problem with my IMac G5. It is surprisingly irritating. LaCie's Silverkeeper backup software was the cause. By default it gave itself permission to wake up the computer to check if a backup was needed. Do you have this or a similar program installed which has scheduled activities including wake-up permissions as an option?"

Dave Hamilton provided this link, xlr8yourmac.com, to a member, which addresses failure of deep sleep with some users that updated and added new FireWire and USB cards.

Also, here is Apple doc.#305262 about PCI cards and sleep.

tomhorvat adds this; After few days of messing around I found out that I had desktop background (wallpaper) set to change every 1 minute.
After unchecking "Change picture..." in system preferences (under Desktop/Screen saver), my powerbook went to sleep all by itself.
Good Luck!  DALE

If unplugging your USB devices doesnt help, it sounds like you have a setting tagged to allow external bluetooth devices to awake your computer.
My cellphone, palm pilot and other bluetooth devices are connected to my laptop and often wake it up randomly. In fact, sometimes my email will download during the night and the computer will wake up to accept it because i have it set to automatic downloads

Here's more on Bluetooth;

A number of sleep-related issues can appear after incremental Mac OSX updates, including the inability to properly go to sleep, problems waking from sleep, and having your display go to sleep without your Mac going to sleep. The most common culprit is Bluetooth. Try these tips to fix your sleep problems.
WAKE UP
In System Preferences>Bluetooth, uncheck Allow Bluetooth Devices To Wake This Computer. If you are using a Bluetooth mouse and keyboard, you'll have to wake your Mac by pressing the power button.
FILE CORRUPTION
Your Bluetooth preference file may be corrupt-you'll need to delete it. Turn Bluetooth off, then go to the /Users/user name/Library/Preferences folder. Look for a file called com.apple.Bluetooth.plist and trash it.The OS will create a new preference file when you enable Bluetooth again.

Some, in these forums, have had luck fixing sleep issues downloading the Combo update from this site (yours may be corrupt), not Software Update, disconnect all firewire + USB devices except keyboard + mouse, Repair Permissions, re-install update, Repair Permissions again + try.
Yours might be the 10.4.11 Combo update (PPC not for Intel Macs). Your specs don't appear to be up to date (says your using 10.4.1). Others reading, download the combo update that applies to you!