If you can find a developer to register your UDID then you can install 3.0. When your phone is connected to iTunes you should see the serial # click on it until you see the UDID instead of serial #. Give the UDID to a developer and have them register it, most if not all will probably ask for a small fee to register you. There are many people on here offering to register people.
Yes, it would be fine, however I would caution you that this is beta software.
They can tell cause you phone will need to be connected to iTunes nd your computer is connected to the internet. I have read you can get away with not being registered if you disconnect your computer from the internet when you do the update and each time you sync/connect your phone to iTunes. No idea if it actually works and definitely seems like a pain to do.