According to a recent New York Times article, the adoption of WiFi is out pacing cell phones. This isn't very surprising as it's a much newer technology, but it's still interesting. The bit about cell phones is located on page 2:
"There are now about 40 million Wi-Fi users, he said, and new access points are selling at the rate of about 15,000 a day, which makes Wi-Fi a much faster-growing technology than cellular telephony."
For carriers like T-Mobile, this is nothing but good news (they power the popular T-Mobile HotSpot service). For brick and morter shops, this is even more bad news.