I was also having this problem of Windows 7 of just going into hibernate on low battery without notification.
I checked out the Microsoft site for Battery faq and how t change low battery notification which didn't tell me something I didn't already know. So I decided to experiment some. I discovered that when I changed my Critical Battery Action to "Shut Down" from "Hibernate", the Low Battery Notification popped up and could switch to a power source. I changed the Critical Battery Action back to "Hibernate" and the Low Battery Notification still pops up. I guess the system just needed a little push :)
My install is barely a week old so I guess the installation didn't make the correct settings. Having applied some changes to the settings has caused the system to behave correctly in this case.
I hope this will help others with this issue.
Cheers