I love the idea. Like some kind of Multiplayer Firefight, except it's much more epic. But there are a few problems:
Instead of just one "Perfect Map", why not build a lot of different maps? Each one would be specialised to certain game types, like a sniper map, a CQB map, a vehicle map... There is no such thing as a perfect map, and people could become tired of the same scenery again and again. Also it could be interesting to make other gametypes to this, and not just the bomb thingy.
Another problem would be the bandwidth. Bungie has never made a game of this size before, and it would need very strong computers. Luckily it is possible. The Resistance games on PS3 have proven it works with over 60 players.
Another problem, as said, is the players camping in their ships, or spawncamping. Instead it would begin on a cruiser in space, where you choose a basic weapon, and then gets dropped on the map with a cinematic ODST-like drop pod scene, dropped randomly on the Spawn Zone. This would make it difficult to camp, since you never know where the pods will land.
But the biggest question is about it drawing too many players form Matchmaking. The solution would be to insert this as some sort of Double EXP Weekend instead, so that everyone can enjoy it but still not steal players from all the other playlists.
Finally, I must say this is a great idea and that Bungie should seriously think about this opportunity. It would become very popular.
[Edited on 11.03.2009 10:43 AM PST]