I have found in most cultures (including the US) that time has different states of focus depending on context. In group social engagements, unless there is an external delimiter (such as the movie starts at 3:00), the focus is very fuzzy -- nobody shows up at 8:00 sharp for a party. In one-on-one social engagements, the focus tends to be a little sharper depending on circumstances. In one-to-one or one-to-many business engagements, the "one" in the driving seat tends to have a much sharper focus than do individual members of the "many" -- a trainer likes to start on time, but the trainees are late; a salesperson likes to be punctual, but the potential buyer(s) have no problem keeping him/her waiting.
Our perceptions of what is "normal" behavior are determined by the habits of our most familiar peer groups. Over the years I have done a lot of work in various Latin American and Asian countries, South Africa, and most of Europe. In a business meeting context, the sensitivity to punctuality is always less cultural than contextual. And within that context you cannot make sweeping statements about national cultural attitudes to time because corporate culture plays a major role in guiding those attitudes. There are a couple of companies that I have worked with in Mexico and Brazil where I am always the last to arrive at a meeting that I am running. Conversely, there are companies in the US and UK where I have given up expecting more than 50% of participants to be punctual, and where participants come and go at will throughout the meeting. Generation and social set also plays into time consciousness. The post-MTV perpetually-looping CNN mode of communication has produced some people who have not only a limited attention span, but who assume that there IS no beginning or end, and believe they can always catch up no matter where they start or how often they get distracted. That fragmentation of attention span seems to be getting worse with the advent of SMS.
Is Internet time overwhelming national culture? As with the CNN comment above, I don't see it on a national scale. The digital divide (if we think of it in terms of those who have embraced connectedness v. those who just get by) is just getting wider. True, "smart mobs" can coalesce and disperse with split second precision. But these are funky folks on the fringe, not mainstream people. What may become more pervasive, particularly as mobile phones become smarter, is a blurring of the line that separates "presence" from "absence". Perhaps technology will be used to inflict punctuality -- you have an electronic window of opportunity to enter this meeting and if you miss it the virtual doors will be closed on you. Or perhaps the technology will make "punctuality" an unnecessary outmoded concept.