With Clearwire struggling to build out a next-generation network that will go up against Verizon Wireless (VZ) and AT&T (T) for future mobile customers, Sprint has little choice but to throw more money at it. But it’s not alone in doing so. Fellow partners Comcast (CMSCA), Time Warner Cable (TWC), Intel (INTC), Eagle River Holdings and Bright House Networks are also making additional investments, though together they total only $500 million.

Oddly absent from this list is Google (GOOG), which participated in a May 2008 investment round that pumped $12 billion into Clearwire but has evidently decided to sit this one out. Said Google spokesman Andrew Pederson: “We have already made a significant financial investment, and at this point we think the best way we can continue to add value is through continued product and strategic cooperation.”

One wonders if the shared mission of which the two companies boasted when the deal was announced has somehow changed in the ensuing year.

The alliance will see the four cable and tech companies investing $3.2 billion in the nationwide wireless network that Sprint and Clearwire have been struggling–with profound unsuccess–to roll out. Comcast (CMCSA) will contribute $1.05 billion, Time Warner Cable (TWX) $500 million. Intel (INTC) will invest $1 billion, Google (GOOG) about $500 million. The new venture will be majority owned by Sprint, but it will take the Clearwire name and be run largely by Clearwire execs, among them cellular industry pioneer Craig McCaw.

For the cablecos, which have yet to settle on a clear wireless strategy, the deal is a quick and dirty way to establish the high-speed wireless network they need to compete with telcos like AT&T (T) and Verizon (VZ). For Sprint and Clearwire, it’s a chance to make their non-starter of a WiMax network viable and something happy to talk about when conversation turns to Sprint’s stock price, which has fallen nearly 60% over the past 12 months.

That said, the deal is not without its problems–top among them WiMax itself. As Craig Moffett, an analyst with Bernstein Research, explained in a note to clients earlier this year, the 2.5 GHz spectrum upon which Sprint and Clearwire are building their network isn’t nearly as good as the spectrum Verizon and AT&T just purchased in the FCC’s 700 MHz auction. “Serious questions remain about penetration through walls and windows,” Moffett explained. “Elsewhere in the world, operators have also raised questions about WiMax’s real-world bandwidth, latency and non-line-of-site coverage. How competitive the offering would be versus Verizon’s or AT&T’s planned LTE broadband service therefore remains to be seen.”

That it does–though there have been some indications that it may not be quite up to par. Speaking at an international WiMax conference in Bangkok in March, Garth Freeman, CEO of Buzz Broadband, Australia’s first WiMax operator, described the technology variously as a “disaster,” “miserable failure,” and a standard “mired in opportunistic hype.”

So will that prove true for Clearwire as well? We won’t know for some time. Building out a massive network like this will take some doing. “We’ll likely to see early trials in 2010, but a full-fledged build-out will take longer,” Clearwire CEO Benjamin Wolff said during a conference call this morning. “Building faster is a matter of logistics. The build plan we’ve laid out will be one of the largest and fastest build-outs ever done. We have the capability to do it, but it’s a massive undertaking.”