Comcast (Nasdaq: CMCSA) just announced a high-speed mobile data service in Sacramento, adding to a list of other cities across California and the broader U.S.

Yes, I just said "Comcast" and "mobile" in the same breath. You see, the company is a major investor in Clearwire (Nasdaq: CLWR) and its WiMAX ambitions. Sprint-Nextel (NYSE: S) likes to steal the 4G spotlight, making it easy to forget about the other investors. And nobody expects the cable company to make a mobile splash, far removed from its hardwired home turf.

But Time Warner Cable (NYSE: TWC) has been selling Clearwire-powered Road Runner Mobile services in select markets as well, so Comcast is hardly alone. Sprint is clearly more invested in Clearwire than the others, both in the sense of owning 51% of the company's shares and in terms of needing the service something fierce. But that doesn't stop other Clearwire partners from jumping on the 4G bandwagon with their self-branded versions of the service.

It must be exciting for Clearwire shareholders to see the company's market and ecosystem growing, even if by baby steps. But I wouldn't expect these tentative moves to make any significant difference to Clearwire's troubled financial performance. Revenue is growing quickly, but gross margins are negative and the company tends to post net losses about the size of its sales.

The 4G advantage Clearwire and its partners have enjoyed is fleeting indeed. T-Mobile already claims to have the largest 4G network in America, with Verizon (NYSE: VZ) expected to announce its first 4G phones at the CES trade show in two weeks, and AT&T (NYSE: T) can't get over the 3G era fast enough. Never mind that none of these companies actually sell standards-compliant 4G services -- it's all in the marketing and the Sprint, Clearwire combination has largely missed that boat already.

Is it too late to save Clearwire from its cash-burning downward spiral? How many more Xfinity Internet 2Go markets would Comcast need to open in order to make a difference? Add Clearwire to your Foolish watchlist to keep track of the company's health -- as fragile as it is.

Fool contributor Anders Bylund holds no position in any of the companies discussed here. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. You can check out Anders' holdings and a concise bio if you like, and The Motley Fool is investors writing for investors.