Is there an aftermarket USB->DockConnector cable that works with an iPhone 4 + bumper case?
Maybe this'll work: http://cablejive.com/istubz
EDIT:
Whoops, maybe not. The 1st party cables are a precision molded plastic that is thinner than the 3rd-party clones as detailed in this video @ 3:40 -
http://youtube.com/watch?v=XpmLvY2XNPg
I just spent 10 minutes in the garage with my Dremel tool, taking off a couple of MM off the plastic around the top of the after-market plugs. Now they fit fine into the iPhone 4 Bumbper case. Problem solved.