I'm trying to develop software that is intelligent wrt sleep events (cleanly closing network connections, making sure data restart locations are set properly, etc).
Are there mechanisms in QT (4.6) currently that facilitate me responding to system power events?
No, nothing like that in Qt no. Probably not and will not. You need to use the API of the operating system.
I dont think there's a native to qt and multi platform way for these but there are propably some ways and api's to do things you are looking for. Posix signals might provide something to notify your app about ongoing system events. Also you might want to look QDbus stuff, some cases dbus will broadcast system events.. But how to do the stuff you are really looking for is very os dependant and without knowning your target os, its rather hard to point out a optimal solution.