This is an issue for me as well.
We have a WPA2 Enterprise wireless network at work with hidden SSID, access to which is via domain credentials. Every 90 days we have to change our domain password and this of course means any wifi access also needs to be changed.
In the past, the only way to update the password on the iPad is to click "Other..." in wifi settings and add in the network settings again from scratch.
Two things annoy me about this:
1 - if the ipad tries to connect to the network with a now-incorrect password it doesn't prompt you.
2 - there's no way in network settings to just update the password.
It would be nice if there was a prompt asking for the password whenever it got updated and then it could go back to remembering the network, but there should at least be a way to change the password in network settings.
Unless I'm completly missing something here.