After a couple of months' development and a lot of fiddling, it's finally available! Pocket Clusters, a collaborative cluster creation tool for the iPhone and iPad. Sorry this took longer than I was hoping folks, but I decided to add some rather funky features. Players sat around the table can now share their cluster via Bluetooth and WiFi so that everyone with a compatible device can download and view the cluster map. Furthermore, the GM can now 'delegate' certain knots to other players so that they can edit the details on their own device, speeding up the process of creating a cluster no end. The edited data is then shared to everyone else at the table.
There's plenty of other features too, such as customisable colour and display themes, saving and loading clusters, and PDF and image output for your finished cluster maps. I tried to make it as comprehensive as possible.
Also did I mention it's free to download? Here's the link:Pocket Clusters on the App Store