Mike,
You did not say what LaCie you got, and if it looking better than the 24" ACD. My guess is that it does look better, and without the glossy screen of the newer 24" ACD. As you are probably doing now, you can profile the LCD on the MBP and have a separate profile for whatever external monitor you use. If you go the black box route, as Jack suggested, the output to that box will be profiled for one monitor, and displayed across both. Not sure that will look great on whichever is NOT profiled. Why not use the MBP's display for the pallets or browser, and just do your work on the LaCie? That way you could have a good calibration for both?
As an aside, I run two 24" monitors off of my G5 and have each profiled. What I have found is that while nice to have all the real estate to look at, the vast majority of the time I am using one or the other for my critical work. That is why I am thinking of dumping both 24" monitors and replacing them with a single 30", maybe from NEC, but would love the EIZO CG if it was a bit cheaper ;-) I am also looking at the new 17" MBP as possibly the "do-most-of-it" machine, and connecting the 30" to it. That would allow me to use the 30" as the main viewing/working screen, while the 17" laptop LCD would be used for browsers or things, though I still find it bothersome crossing from one to the other. Just my thoughts on this, as I have not found an ideal solution myself yet.
LJ