Have you guys never heard of Krusty buckets? Multiple HPS (1000 watters) hung vertically in between 4 or 5 plants in a modified bubbler. Yield close to a pound a plant, if I recall correctly.

In short, yes, hps can be hung vertically. But, as Stinky said, you have to be careful that your specific bulb can be hung that way. Most modern hps bulbs are now universal. MH bulbs you got to be sure.