if you're thinking of developing a web app in lisp but haven't looked at python or ruby, you might consider them -- remember, it's not just the raw language but also the surrounding infrastructure, and python (django, turbogears) and ruby (rails) have arguably much more developed frameworks, and also arguably have most of the interesting language features that would draw you to lisp in the first place.

the point is the language is only one piece of the equation -- you want to be spending your time developing your app, not rolling your own supporting elements (templating systems, init scripts, form validators, sql bindings/mappers, etc.) and of course i'm sure others will point out reddit's switch from lisp to python.