I basically do the same as WSmith, throwing in some NPC rolls for good effect. Large battles can be done in not too much more time than regular combat (which can be slow anyways).
The largest I've run was an army of 500 Orcs against a town of 500, with @80 Guards/Soldiers/Rangers/local NPCs, 16 NPCs (two other parties) and 6 PCs. Took a good 2-3 hours, but the players had a huge impact on the battle. Fun when the group Cleric went down fighting the Orc General one-on-one, and several others had to spring to her rescue. One Heal spell while she's being protected, then she jumps back into combat and kills the bugger. Meanwhile the Ranger led a charge that wiped out a breakthrough on the other side of town. What more could you ask?