by some estimates, whopping 5,000% mark-ups.

These plump profits may, however, be on the wane, at least in the increasingly wired rich world (text-messaging continues to thrive in developing markets). Apple's new iOS 5 operating-system upgrade for its ubiquitous mobile devices includes a revised Messages app that automatically switches to a 3G or Wi-Fi data network to carry text and multimedia content if it detects that the recipient's device is also running iOS 5. It works across national borders, too, avoiding roaming charges, another source of juicy margins.

Apple will not be the first company to take this tack. Research in Motion (RIM), the maker of BlackBerry smartphones, introduced device-to-device messaging, called BlackBerry Messanger (BBM), as part of its basic service in 1999. At the time few Americans had heard of text-messaging, and carriers did not see RIM's new-fangled BBM as a threat. Even when texting took off, expensive BlackBerrys were regarded as the preserve of businessmen. Teenagers were still busy thumbing thousands of traditional text messages each month on cheaper mobile phones.

Since then, smartphones have spread beyond corporations (few besuited road warriors were spotted among the BlackBerry-toting London rioters in August, for instance). This prompted operators to start offering unlimited messaging plans, eliminate lower-volume texting options and raise the price of a single message. Currently, unlimited plans go for around $20 per month per person, or $30 per month for a family. The alternative is to pay 20 cents for each message with no plan; there is little in between.

Unlimited plans comfort subscribers who fear sudden gushes of overage fees when they exceed metered text plans. They also lock in profits for the operators. Yet they are being made obsolete by smartphone apps for Facebook, Twitter and the like, which are fast becoming an outlet for messages that would once have gone out as SMSs. Google offers free texting through Google Voice, available on Android and iOS, though the service requires separate registration and setup.

The iMessage, by contrast, is a paragon of seamlessness. No additional software or fiddling with settings is required. In testing, Babbage simply selected a contact's mobile number and, if the contact had already upgraded to iOS 5, greyed-out indication in the text field would fade out "text message" and fade in "iMessage". Users can register various e-mail addresses through which to route the messages. Like BBM, iMessage also confirms delivery and optionally allows notice of a text being read.

All this means that, as often with Apple's products, iMessage may prove even more disruptive. It hits an enormous user base: Apple says it has sold 250m iOS devices, including some 140m iPhones. At least 175m more recent iPhones, iPads and iPod touch devices can be upgraded to iOS 5. They represent, it is true, a fraction of mobile customers worldwide. Nevertheless, iMessage's impact could well prove disproportionate. For a start, it is likely to replace texting for some of the most avid texters. And even if Apple ultimately decides to make iMessage a proprietary service, it may compel operators to allow Google and Microsoft to offer similarly frictionless text replacement. The days of 5,000% mark-ups may be numbered.