Whispering in other cultures

Whispering in general, no - it's ok to keep your voice down to avoid disturbing others (eg during movies, watching live golf, tennis, etc).

However, some people do certainly consider it rude to whisper in front of others. It's akin to talking to one one of two (or more) people in a room as you're excluding them from the conversation.

Are there other cultures where whispering to avoid bothering others is the correct thing to do?

I would say, in most European and Asian countries keeping your conversation down to the level that people around you are not bothered with it is the correct thing to do. This is especially true in mass transit. In "quiet" subway cars, for example, ANY kind of conversation is frowned upon and would give you weird looks (you'll see lots of people around texting though).

Are there other cultures where whispering is taboo?

I'm not aware of any, and this is certainly not the case here in America. And I agree to what phoog and choster said. Indeed fellow Americans are usually those loud people you can hear from across the street.

I am from the Netherlands. From a young age I was taught that it is impolite to whisper when there are other people around. Now I am in Indonesia, and the youngest child (of 12 years old) of the family where I am staying whispers a lot to his older sister (about 20) when I am teaching them English. It annoys me, but I have restrained myself from saying anything. After reading the comments here I think I'll go on trying to restrain myself. But if you ever go to Holland, remember that it's a definitely 'not done' there.