Mobile virtual network operators (MVNO) don't own a physical cellular network. Rather, they rent and resell spectrum from 'real' wireless operators as voice and data services. The most high-profile recent MVNO appeared when The Walt Disney Company launched the family-friendly Disney Mobile service - with tight parental controls - over the Sprint PCS network during the middle of last month.

Some other, slightly older - but also high-profile MVNOs - could be in trouble, however.

The Wall Street Journal reported a couple of weeks ago that MVNOs Mobile ESPN and Amp'd have fallen well short of expectations. Amp'd rolled out in December and Mobile ESPN in February. Both, with combined financial backings of over $300 million, didn't even hit 10,000 subscribers through May.

Amp'd is a youth orientated lifestyle MVNO with a limited three handsets to choose from, while Mobile ESPN launched with a re-branded (and not-so-handsome) Sanyo MVP mobile phone, which nonetheless offered a convenient "E" launch button so users coulg gain easy access to sports news, video clips, games, fantasy teams and more sports related content. A second phone, a thinner Samsung Ace (also known as th A900), finally joined the Sanyo a week or so ago.

To get things moving in the right direction, Mobile ESPN is headed back to the drawing board, while Amp'd is going to continue as is, marketing to youngsters, for the foreseeable future to boost ratings. You'd think the powers-that-be would give Mobile ESPN the football season to prove itself?

What could be even more startling to some, is a rumor that Helio, the MVNO partnership between Earthlink and South Korean mobile operator SK Telecom, isn't doing so hot either. Helio targets tech-savvy 18 to 34 year old through a partnership with the 87-million strong MySpace social-networking site and a pair of cool-looking and functional smartphones called Hero and the Kickflip.

According to the blog engadget, an unconfirmed report from Telecoms Korea states that as few as 100 people have signed up for Helio; a staggeringly low figure considering how much promotion went behind the MVNO's launch.

The MVNO denies this number is accurate, however. But it wouldn't give away the real subscriber figures.

Helio plans to expand its handset lineup with up to five additional models year. And it has recently implemented an electronics trade-in program; whereby subscribers can send in an old PDA, cell phone, digital camera, gaming system, or MP3 player to receive not a discount or anything like that, but cash (a check in the mail really). Not much money, it seems, but something to get hipsters excited.

As of April, MVNOs accounted for around 5 percent of the total U.S. mobile market, but that number could rise to as high as 10 to 20 percent in the next few years, according to Gartner analysts.

In a survey of 1,017 wireless subscribers, In-Stat reported earlier this year that 80 percent of respondents said they'd be willing to buy service from an MVNO. Folks appeared readier to switch from one of the smaller operators than one of the big four, who happen to be the carriers renting out the spectrum to the MVNOs anyway.

Thus making virtual carriers even less of a potential problem for Cingular, Verizon, Sprint and T-Mobile than any other traditional American operators.