Clearwire also said it had secured an additional $1.56 billion to continue its network build out from tech heavyweights like Sprint Nextel (S) – which owns a majority stake in Clearwire – Comcast (CMCSA) and Time Warner Cable (TWC). Clearwire's success is especially important for Sprint, which just said it would invest $1.2 billion of the funding and is struggling to stem losses to bigger rivals Verizon and AT&T.

Morrow said that Clearwire was in the "right place at the right time" to take advantage of the boom in wireless data usage. But investors aren't buying the upbeat posture. Shares sold off more than 15% on Wednesday to close at $6.14.

Clearwire faces a long list of challenges as it attempts to capitalize on the growing consumer shift to wireless data. And while daunting in itself, securing the additional $1.4 billion funding gap that analyst Michael Nelson at Soleil Securities estimates the company will need by 2013 may be at the bottom of the list for Clearwire.

Clearwire's 4G technology is based on a standard called WiMax. Pushed by Clearwire investor Intel (INTC), the standard is further along in the development process and generally cheaper to make devices for than that of telecom rivals. But Clearwire's service has been plagued by reports of glitches like frequent outages and slow bandwidth speeds in the cities in which it currently operates.

Telco giants Verizon and AT&T, meanwhile, are now on the cusp of rolling out their own 4G technologies based on a competing technology platform. While the Long Term Evolution platform took longer to develop, it is considered more robust and powerful than Clearwire's.

Clearwire had banked on getting into the market ahead and tying up customers of its telco rivals. But giant Verizon – largely seen as operating the best 3G network and known for spending top dollar on its networks – is getting more aggressive. Verizon is reeling from the popularity of AT&T's iPhone and has identified building out a faster network as a cornerstone of its strategy to retaliate. While Verizon was already expected to make a big push in 2010, that build out may be about to get more aggressive.

Morrow downplayed the threat from rivals, saying that Clearwire didn't have to "unseat any of the giant telcos in order to be successful."

But even some of Clearwire's savviest prior backers seem to be veering towards its telecom rivals now. Google (GOOG) – which is banking on the boom in the wireless sector to power mobile ad growth and picked up startup AdMob for $750 million this week – had put in $550 into Clearwire a year ago. But the search giant abstained from the current round despite its massive balance sheet and is instead focusing on its Droid partnership with Verizon.

So it might be a case of too little, too late for Clearwire – and that is if it manages to lineup future funding for its network built.