The authorities in some countries (like the USA, for example) can tap your telephone, if they think they have a good reason to do so. Skype calls are encrypted and persumably cannot be easily tapped by the authorities.

My question: is it legal to use Skype in countries like the USA, where the authorities might like the opportunity to listen to certain conversations?

in my opinion this question is the same as asking "is a private conversation legal in countries like the USA......"

my answer is most certainly.

in countries such as the USA there is no law to say stop you sending a handwritten sealed note to a friend.... better yet, there is no law to stop you sending a message to a friend in a code only you and that friend understand, be it a mathematical code, a code known to other countries but not the USA such as an almost dead language, the point being private conversation is allowed.... skype messages are conversations, the encryption is simply another language, one which is difficult for anyone other than the users of that language to understand. so to say the encryption of communication should be banned is to ban a language... i guess a government can decide to do that, but i doubt there'd be much support for it and i doubt that government would last.

so it most cretainly is legal to use encrypted messages, such as those transmitted via skype.

There are some countries that have outlawed VoIP services (presumably to protect local telecommunications industry). Countries in the first world generally have very protective laws or legal precedents to protect rights of privacy and communication, but these rights are not universal (even though they ought to be). I am quite sure that Skype would be legal in the US, Canada, UK, Australia, New Zealand, and most (if not all) of the rest of western Europe but you really need to check your own government's laws to be sure.

right i'd forgotten all about them... oops,

well the law's in those countries are silly in my opinion

It's definitely legal. If it weren't, then Pretty Good Encryption (PGP) wouldn't be legal either.