I was going to make the same comment. Frankly, I barely find gdb to be adequate for debugging iOS code (lackluster support for Obj C 2.0 properties, etc). The idea of having to debug a significant amount of JS inside of a native app shell without a real debugger gives me chills.
The big hope is that XCode 4.2 reduces the overhead (i.e. suck less) of Objective-C through LLVM & LLDB's improved debugger, more static checking and automatic reference counting.
