Each somehow popular web applications needs one: A proper web installer, there still is no general solution for that. Christian Weiske already blogged about this issue some time ago. He also came up with a list of requirements, which sound very sensible. Sadly the pear installer does not provide a way to build such web-installers in an easy way, without shell access, and additionally pear is often not even installed properly at mass hosters systems.
Since implementing an installer for Arbit is one of the bigger points on the current roadmap, we are facing this situation right now. As you can see from the different libraries we already spun out of Arbit, we try to do such things right, once we need them. So we started a design document, based on Christians requirements, to specify what we need from an installer and how it could be implemented.
I founded Qafoo together with two other Arbit contributors, and beside consulting and training around high quality PHP software we provide commercial support for Arbit and other software quality related tools.
If you are interested in a web installer for your distributable application, I would love, if you could check out the design document and verify that it meets your requirements, or is extensible enough to do so.
If you have to add anything to add or discuss, please join #arbit on FreeNode to get in touch with us.