While I didn't completely replace my desktop like what you're proposing, I did augment my workflow with a 15" i7 MBP(mainly do LabView development and the like) and I did experience any slowdowns and I actually don't use my desktop that much anymore unless I intend to game a bit. To be honest, I think it puts you at an advantage depending on your work situation. The ability to carry your work is an excellent way of going especially when you need a change of scene from those long coding sessions! Just make sure to pickup something like LoJack or undercover, lock down the firmware and get a good kensington lock and bag and you're good to go! Hope this helped!
For reference: I run an Xeon X3520@ 4.2Ghz, 16GBs of DDR3 RAM and a GTX 285 in my desktop, just to give you an idea of what I was coming from.