According to Wireless Intelligence (subscription required), the global number of cellular mobile telephone subscribers surpassed two billion this weekend. Of course these are estimates -- it will be a few years before we have more solid data. But it's credible. Wireless Intelligence is a joint venture of the market research firm Ovum and the GSM Association. As a trade association, the GSM Association may lean to an optimistic view, but Ovum is a market research firm that I've had experience with, and consider independent and credible.
We only reached one billion subscribers in mid-2002 (20 years after the first analog cellular telephony trials), so it's pretty impressive to pick up the next one billion in only three years.