How can I run a script when my network connection changes?

You can do it using Task Scheduler:

1. Right-click on the "Task Scheduler Library"
2. Click "Create Task"
3. For "Name:" field write Your new task's name.
4. Check that the "Security options" area has checkboxes where You want them to be.
5. Under "Triggers" tab click "New..."
6. Select "Begin the task:" => "On an event"
7. Select "Log:" => "Microsoft-Windows-NetworkProfile/Operational"
8. Select "Source:" => "NetworkProfile"
9. Write "Event ID:" => 10000
10. OK this dialog.
11. Under "Actions" tab click "New..."
12. Select "Action:" => "Start a program"
13. For "Program/script:" browse to Your favorite script file.
14. OK this dialog.
15. Setup the "Conditions" tab regarding "Idle", "AC power" etc conditions to Your liking.
16. Also do setup under "Settings" tab.
17. OK, all done here.
18. Test the new event.

Just in case, there are also events for computer "Suspend" and "Resume" available there. I, for example, use them to stop and restart Hamachi on these events, since otherwise it seems to have problems on my machine.
For computer "Suspend" You can use Log "Microsoft-Windows-Kernel-Power/Thermal-Operational", Source "Kernel-Power" and the Event ID is 42.
For computer "Resume" You can use Log "System", Source "Power-Troubleshooter" and the Event ID is 1.

For user login/logoff, I do not know events under Task Scheduler (but I believe they are available there too), but You can configure it instead here:

1. Launch gpedit.msc
2. Open "User Configuration"
3. Open "Windows Settings"
4. Click "Scripts (Logon/Logoff)"
5. In the right pane You can assign scripts for "Logon" and "Logoff" events. Note, if You assign multiple scripts then do not assume that they are run synchronously. They may not. For this case use single script instead, that calls other scripts in sequence. I haven't checked under which account these scripts run - is it user or system?

You can do similar stuff for computer "Startup" and "Shutdown" events under "Computer Configuration" folder in the same window. The Startup script runs under System account before the user logs on.

The "Network Location Awareness" service (http://msdn.microsoft.com/en-us/library/ms739931(VS.85).aspx) in Windows XP (and 2000? I don't recall...) and up will enable this functionality, but I haven't found where anybody has written an application to take advantage of it. I'd love to code something myself, but I don't have enough spare cycles to even begin to think about it.

This wouldn't be a simple little VBScript thing, but it wouldn't be that much coding either. Maybe somebody could pick up the idea and run with it. There's even sample code at http://www.microsoft.com/downloads/details.aspx?familyid=ef8a6228-f11d-4ba0-b73e-dd8dc7dd11e8&displaylang=en.

There have been numerous times I've wanted this functionality, and I'd think there are more than a few people who would like to see it.

I've had some success with Net Profiles before.

Using .net framework version 2 it is possible to use the System.Net.NetworkInformation namespace. From this you can determine:

network availability: NetworkAvailabilityChangedEventHandler() network address changes: NetworkAddressChangedEventHandler() ipaddress: IPAddressInformation

and so on.

How much of this is available through powershell I don't know because I don't use it. But I have written a very simple vb.net tray application that just monitors network availability and connects to a network share when the network is available. If you're interested in the bare-bones of the code then let me know.

This is kind of a sideways and partial answer (and not even documented because I can't seem to find the original article), but I'm fairly sure Windows 7 has a property sheet on the printer object allowing you to change defaults depending on subnet.

I'd love to see features addressing the rest of your list, as well.

You could create a batch file that reads the results of an ipconfig command, and have it run every 5 seconds or so...