Has it become OK just not to respond to email invitations? When I'm traveling to cities, I'll email friends and acquaintances (individually, not as a group) with "I'm going to be in your city for a few days and if you're free I'd like to meet you for a drink or dinner or something." When I don't hear back, I have no idea what to do.
It's never Perfectly Proper to leave an invitation unanswered. Etiquetteer sympathizes with your plight. Vagueness, however, can make it more difficult for someone to think it's important enough to respond (and indeed, they might just respond "What did you have in mind?" which is even more vague about their availability). So Etiquetteer recommends more specificity. In your email, suggest a date and a location, adding "If another time and place works better, please tell me." Even more important, give your friends a deadline by which to let you know: "If you could let me know by (three days before your suggested date) it would be great. I'll check in with you closer to the time if I haven't heard fromyou." This way you set the expectation that their response is important, and that they'll hear from you until they send it.
Or, until your deadline, after which Etiquetteer absolves you of any conflicts. It's Perfectly Proper, then, to say that you made other plans after not having heard back, but say so with a tone of Infinite Regret.