The options for getting mobile broadband more cheaply for your laptop, iPhone, and 3G iPad keep multiplying as carriers want to take advantage of a growing market. Virgin Mobile joins the fray with a $40-per-month, 30-day service plan with unlimited usage that's now available.

Just two months ago, Virgin Mobile - a division of Sprint Nextel - began offering the MiFi cellular router without a contract for $150. Service plans started at $10, and worked with either the MiFi or a USB modem compatible with Mac OS X and Windows. (See "Virgin Mobile Offers MiFi Mobile Hotspot without Contract," 28 June 2010.)

But the plans topped out at the same rate charged by carriers for laptop plans: $60 for up to 5 GB used in a 30-day cycle, and there was no automatic renewal or billing. In my article at the time, I explained how the flexibility of Virgin's offering could be the right combination for someone travelling with one or more iOS devices and a laptop, rather than using AT&T's iPhone iOS 4 tethering ($45 per month including 2 GB of usage), or a MiFi from Sprint or Verizon, which requires a two-year contract.

This new $40-per-month deal is extraordinary, and Virgin Mobile lists no restrictions on its offer page. This seems to be a terrific alternative to iPhone tethering or 3G iPad plans.

The Virgin Mobile MiFi connects to Sprint Nextel's 3G network, which operates more slowly than AT&T's top rates in principle, but isn't that far off in practice. AT&T has plans to double its raw 3G rates by year's end. (Sprint has a far faster 4G network, run by its Clearwire division, that reaches about 50 million people.)

AT&T requires that you have its 2 GB DataPro plan ($25 per month) to use tethering ($20 per month). The DataPro plan costs $10 per 1 GB used above 2 GB in a 30-day billing cycle. The cheaper DataPlus plan, at $15 per month for 200 MB of data, costs $30 less than the DataPro with tethering option. (AT&T does let you switch your service plan level and turn tethering on and off at will, instead of requiring a new contract or other limits.)

Beyond the limited service, iPhone tethering is also irritating in comparison to using the MiFi, requiring the right combination of elements to make it work. If you want to use more than one device, you have to employ a wonky workaround (Internet Sharing in Mac OS X set to share the tethered connection). The MiFi can share its connection with up to five Wi-Fi devices at once.

Sprint's 3G network currently has about the same coverage as AT&T, reaching roughly 80 percent of the U.S. population. If you're out of Sprint's area, the company roams you onto Verizon's network, but limits you to no more than 300 MB of Verizon service in a billing period - something that's nearly impossible to know in advance.

Virgin Mobile doesn't note this limitation, and says it uses only the Sprint 3G network, which may mean you could be without coverage in Verizon-only territory.

But Sprint has coverage in all major and medium-sized cities, as well as at airports. The tens of millions of people living in smaller towns and exurbs that are served by Verizon with 3G but not Sprint are the ones who would encounter this particular dearth.