There are ways to allow two or more devices to share a 3G cellular backhaul link to the Internet. The main motivation to do this would be to avoid the cost of a 3G network interface card (NIC) and a 3G data service plan for every device by letting two or more devices share one connection and data service.

The most prominent options are cellular tethering and MiFi services, which link multiple local devices to a 3G-enabled smartphone via Wi-Fi.

For its part, tethering allows the handset to serve in two roles: phone and modem (effectively, a cellular NIC). So you buy one device instead of two. However, one tradeoff is that you cannot make voice calls while the phone is in modem mode.

Tethering via Apple's USB dock connector or Bluetooth is one capability of the recently announced Apple iPhone 3G S, though there is a mysterious delay in AT&T's support of the capability in the United States. AT&T, which already offers tethering on 11 other smartphones, has publicly stated that it will support tethering iPhone 3G S, but has not yet said when.

Unfortunately, shared 3G links won't be available using the iPhone's embedded Wi-Fi connection -- unlike MiFi services by competitors Verizon Wireless and Sprint -- and Apple warns that additional cellular fees from the carrier might apply. AT&T's other tethering plans are $60 a month for BlackBerry and $65 a month for other device and include 5GB of usage.

MiFi services from Verizon Wireless and Sprint entail a user carrying a $100, credit card-sized portable wireless router to which as many as five or six devices can connect. From there, the devices -- such as laptops, cameras, gaming devices and multimedia players -- share 3G data network access (EV-DO or HSPA) to the Internet.

These plans are also around $60 per month. Again, though, cellular data usage is capped at 5GB, and overages cost 5 cents per megabyte. 5GB should be plenty for an average business user sending and receiving e-mail and working with some files.

But my longtime colleague, Steve Taylor, principal of the Webtorials broadband networking educational site, says to be cautious about wanton usage. He says he ate up about half a gigabyte watching a movie on the Verizon Wireless network and is "afraid" to use his services for this application because it's difficult to track usage.

Joanie Wexler is an independent networking technology writer/editor in Silicon Valley.