Firstly my home box I have been hitting the power button since it is quick to resume and I don't really worry about things. I always just thought it was putting it into sleep, well this weekend I stumbled across a new feature in Vista (well new to me) called help. It's amazing, it's like Google for windows off line
Seriously, in the help feature I found an article on what the power button does. Basically if you have an old machine the power button puts it into sleep. However if you have a more modern it puts it into hybrid sleep. This new mode, works like both hibernate and sleep in shutdown (i.e. it does both). Then when it needs to start up again it checks to see if the memory still contains the information (i.e. sleep mode) and if it does restores damn fast from that. If it doesn't contain (say due to power loss) then it restores from the hibernate information.
The power button is the ONLY way in the Vista interface to do this!
To check if you can use this or to turn it on/off go to power options (either off the battery icon in the task try or in the control panel), click Change Plan Settings on your selected power plan, next click Change Advanced Power Settings and under sleep you will see the options (see the attachment of this post for a screen shot).