Sometimes Mac OS X 10.6 Snow Leopard can be slow to sleep. This can be annoying, since if you're putting a notebook to sleep, it's likely that you want to get going and quickly. A post on the Apple Toolbox blog points out a number of culprits and remedies — the list ain't short.
The article says users should check whether there are print jobs stuck in the queue, disconnect or unmount external storage devices, and check for hung processes.
Comments to the post point out another common problem that can affect sleep speeds: Safari and other open browser windows. I have found this to be true. I try to remember to close Safari before clicking the Sleep command.
About a year ago, I posted about the different kinds of sleep available, as well as, the SmartSleep preference pane and an Apple support note on sleep. Other problems can include Bluetooth settings and applications that are wrongly calling the IORegisterForSystemPower() and IOCancelPowerChange() calls when receiving a power management notification from the system.