is it dangerous to run gtk-gnutella in ultrapeer mode with the selected port open to the world or should I be completely behind a firewall?
At the moment all ports are closed except 7777 (for games) and 8124 (for gtk-gnutella).

edit: It just switched back to leaf mode, I don't know why it did this, this was the first time I had seen it run in ultrapeer mode after months of use.

At the moment there is no assurance gtk-gnutella cant be exploited. That is one of the reasons why gtk-gnutella does not allow you to run it as root.
However, if you allready have a port open for gtk-gnutella, you might as well run it in ultrapeer mode, it won't mean any aditional security risks.

That puts my mind at rest! many thanks

For what it is worth, my university almost shut my computer off from the world due to the fact that while in ultrapeer mode, the large amounts of icmp packets set off their intrusion detection system (IDS). Not that it was doing any harm to anyone. Mind you, being a computer in an academic lab, the files were academic related and not files that would have offended the so called copyright ownership of certain larger media companies. But it does point out the fact that ultrapeer mode may perhaps be more visible if that is an issue for you.