This double XP applies across all maps and modes and is presumably for all platforms (PC, PS3, Xbox 360). The bonus XP will show up at the end of each round as the "additional points" that you may have already seen previously during your play sessions.
This double XP only applies to ranked servers. Due to the nature of how the points come at the end of a round, it also doesn't seem (at least not to us) that this double XP will help you to unlock weapon upgrades any faster.
Know what else has a double XP weekend this weekend? Modern Warfare 3. Interesting.