Personally I think they are going to make the FMV's real time, or at least most of them... but then again, it really depends on how much space the game cards can really hold. Nintendo says "over 1 gigabit" which is 128 megabytes. In comparasin, Mario 64 on N64 was 4 megabytes, Perfect Dark and Ocarina of Time were 32, Conker was 64 (the biggest N64 game, along with a couple others), the GBA pokemon and FF tactics games are 16.... GBA carts can hold about as much as N64 carts, as in, anywhere from 2 MB to 64 MB, I'm guessing the DS cards are up to 512 MB, maybe more depending how much you can really get out of the technology. Most retail CD's are 640 MB, and DVD's are thousands of MB.
Oh, and it wouldn't surprise me if it shipped on more than one game card. The DS has enough RAM to keep a game running while you switch game cards, and if the RAM still isn't enough it could ship on two DS cards and a GBA cart, with the GBA cart being permanently inserted and holding the most crucial game files and keeping your game saves, so that the DS cards can be switched with the GBA cart keeps the game running. This of course depends on if the DS can read both drives at once. To a lesser extent, it also depends on if it can detect a new game without restarting. I know if you load up the main menu for the DS and switch games it won't recognize it, but this can easily be solved in software. card 1 just needs to put coding into the RAM that will allow the DS to realize there has been a game swap.