Easiest way to get charms en masse if you have decent gear:

1) Invite an AFK friend from a major city to join a raid group.

2) Head into Ulduar, 10 or 25 works just fine.

3) Grab a Demolisher or Siege Engine - different players will have different experiences as to which vehicle they prefer. I prefer siege engines, but YMMV.

4) Go down the ramp, and kill dwarves. Do not destroy towers, and do not go past the first or second row of towers unless you want to die to Helicopter/Colossus aggro.

5) Kill lots and lots of dwarves. Hope you emptied your bags first!

I have been clearing somewhere in the range of 2-3 BRACELETS per minute with this method (that's 20-30 charms), since the event went live. It is stupidly simple, and you'll have more charms than you know what to do with.

As of Feb 9, killing mobs on vehicles no longer gives charms.

Level 80 bombing quests are being reported as a great way to farm for the charms. I believe there are a few of these in Icecrown that are worth looking into. The mobs that are the targets for these quests are often easy to kill en masse even if not in the vehicle.

http://www.wowhead.com/?npc=35092

Using a siege engine in Ulduar and killing dwarves in the first area also looks to work.

The very weak yellow zombies in Culling of Stratholme are another option.

Wowhead comments are full of discussion about good places:

Create Lovely Charm Bracelet comments: http://www.wowhead.com/?spell=69531

Lovely Charm comments: http://www.wowhead.com/?item=49655

While there is no exact area there are quite a few areas in Icecrown where there are tons of mobs that you can AoE down, assuming you are a death knight or paladin. Other than that just go around killing any mobs that you feel like as long as they grant XP/Honor. My recommendation if you are a little low on cash is just do various daily quests, and get the charms from those mobs.

since you can no longer get them while on a vehicle, i would recommend going to sholozar and doing mass pulls on frenzyheart or oracles, whichever hates you. they have less than half the health of regular mobs and can be aoe'd down very quickly. they also have quick respawns.

For any level 80 character, there's a very easy way to rack up the lucky lovely charms.

Grap the Battle Before the Citadel quest [your faction/class equivalent, or the Valiant's equivalent for a faction.

Go to the citadel, mount up, and just run over the skeletons. In about 10 minutes, you can rack up about 100-150 of the charms

Edit: As of reset on the 9th/10th of February, this tactic no longer works.