NEXTEL OFFERS WIRELESS DEVICE: Nextel Communications, best known for its cellphones with the walkie-talkie feature, has rolled out its first "air card" service offering wireless access to laptop computers, which at least for now can claim wider coverage than some of Nextel's biggest competitors. Nextel's iM1100 device sells for $ 350 with a $ 55-a-month unlimited usage plan. It has its own battery, so it reduces the power draw on a laptop battery. It promises access at speeds of 20 to 56 kilobits per second, comparable to a landline dial-up modem. The Nextel unit works any place Nextel phone service works, in contrast to Verizon's Express Network, which so far has been launched only in the Boston-to-Washington corridor, Salt Lake City, and parts of Northern California. Sprint PCS is planning its own rollout within weeks of a service comparable to the Verizon wireless data network, which promises 40 to 60 kilobits per second.