Is it possible to use the Samsung Galaxy Tab as a wifi-hotspot so that non-3G iPads can use the Samsungs connection to the phone network?
The Galaxy Tab comes with Android version 2.2 (Froyo), this version of the Android OS comes with the ability to work as a WiFi hotspot for up to 5 devices built in. This lets you share your Tab's 3G connection out to other devices over Wifi.
I've tethered wifi only devices like my Kindle 3, and a wifi Skype phone to my Galaxy S's wireless hotspot in this way and connected them to the internet with no problem, so I can't see any reason you'd have trouble doing that between a 3G Galaxy Tab and a Wifi iPad.
(it is possible for your phone company to block this facility, to date I think Verizon in the US are the only company to block it)
Yes you can.
Settings > Wireless and Network > Mobile AP
This feature is built by Samsung, and different from the tethering feature that comes with Android 2.2 (Froyo). Using this one you can turn your Tab as a password-protected hotspot