I had an incase slider on my 3G and the proclip adjustable holder fit it like a glove.
So the question remains - has anyone successfully used an iPhone 4 with an incase slider case on it and had it fit in a proclip holder without removing the case?
My brother has several of these for his previous iPhones and other miscellaneous devices (GPS, satellite radio, iPod Touch) and swears by them. I myself, am really looking forward to the upcoming models that also support the bumper.