I know that this topic isn't exactly brand new, but so far I had no reason to fight with it myself. When Intel released the first new Pentium III processors with on-die 2nd level cache, code name 'Coppermine', most of the processors came as Slot1-versions and only the 500 and 550 MHz models were released for the 'new' FC-PGA 370 Socket.
This Socket doesn't really come across as that new, because it's physically identical to the Socket370 used by all Celeron-processors available today. Technically there is a serious difference though, which is why it is impossible to run the new Socket-Coppermine in old Socket370-motherboards that were designed for Celerons.
FC-PGA Coppermines Now Up to 750 MHz
Recently Intel made a major step towards this FC-PGA-Socket and released clock speeds up to 750 MHz in this package, so that even the most performance hungry people might consider going for this version rather than the Slot1-Coppermine.
I didn't really want to have that much to do with this FC-PGA ('flip chip pin grid array') in the recent months and used happily the good old Slot1-Coppermines in SECC2-package.