I've gone back and watched it a couple more times and there are definitely small hiccups in the framerate throughout the video. I notice it primarily when wide-screen panning is occurring. They are minor, however, and not something that would detract from gameplay (if these were actual gameplay instead of a cut-scene). It could be video artifacts as well, but I believe they are in the actual intro itself. Again, they are small and you have to be looking for them.
In the end, it is an intro. If it does have a couple of framerate hiccups it is nothing to worry about unless they carry over into the game itself and actually affect gameplay. I guess we'll see in a week, won't we?
I'm very sensitive to framerate and tend to really notice when things change -- even slightly. I highly doubt, however, that GTAIV will have noticeable slowdown and the game will rock (for those that like the genre).
I hate trolls.

Systems I currently own: 360, PS3, Wii, DS Lite (2)
Systems I've owned: PS2, PS1, Dreamcast, Saturn, 3DO, Genesis, Gamecube, N64, SNES, NES, GBA, GB, C64, Amiga, Atari 2600 and 5200, Sega Game Gear, Vectrex, Intellivision, Pong. Yes, Pong.