On the same day it started taking pre-orders for the iPhone 4, Verizon Wireless warned customers it may clamp down on heavy users of its wireless data services.

A document on the company's website said that subscribers who sign a new contract for an unlimited data plan, which is required in order to get a Verizon iPhone, are agreeing that the carrier may throttle their data speeds if they use so much data that it slows down service for other users. The document suggests that Verizon Wireless will start slowing down download speeds for the heaviest 5 percent of its smartphone data users.

Verizon is the second company to offer the iPhone 4 to its customers after AT&T. One advantage of buying the Verizon Wireless iPhone was its inital offer of unlimited data services for a flat monthly fee of $30. AT&T eliminated its unlimited data plan last year.

The company may also compress video feeds to smart-phone subscribers in order to save network bandwidth.

T-Mobile USA already throttles the speeds of some heavy smart-phone users.

During the first day of online sales of its new iPhone, the Verizon appeared initially to be facing heavy demand for the phone on its website, as some who tried to buy it were instead stymied by error messages.