Regarding Sprint Nextel's (NYSE:S) $2.2 billion proposed deal to buy Clearwire (NASDAQ: CLWR), remember this old adage: Twixt cup and lip, there's plenty of slip. Just ask AT&T (NYSE:T) about its failed $39 billion takeover bid for T-Mobile USA.

What seemed like a pretty sure thing to AT&T -- even with its army of lawyers and lobbyists swarming Capitol Hill -- failed to clear regulatory vetting by the Federal Communications Commission, and even evoked an antitrust lawsuit from the Department of Justice.

The call to arms against that merger had to do with it creating an uber mobile carrier, surpassing Verizon in number of subscribers and leaving Sprint much much further behind. That AT&T-Rex of a company, it was felt, would have upset whatever balance of wireless competition there was left in the country.

Slip No. 1
Even though the buyout agreement Sprint made with its longtime WiMAX network provider Clearwire would not seem to raise the hackles of the regulatory bodies by creating a super-sized mobile operator in terms of customer numbers, it may do so by the sheer amount of wireless spectrum it would put under Sprint's command.

According to a Citibank study, in 2010 Clearwire had 133 MHz worth of spectrum licenses under its mattress and Sprint had 51 MHz worth. That 184 MHz total is more than Verizon and AT&T have combined. If the AT&T merger with T-Mobile had gone through, their total would have only been 125 MHz.

The founder of Recon Analytics, Roger Entner, wrote that Sprint's and Clearwire's combined spectrum would, if approved, account for one-third of the amount of usable broadband spectrum licensed by the government. With roughly half the amount of subscribers of either AT&T or Verizon, that would give the new Sprint a huge potential capacity and speed advantage once its network infrastructure is built out.

Slip No. 2
Despite the seeming advantages for Sprint in buying out Clearwire -- and the boards of both companies voted unanimously for the agreement -- some Clearwire investors clearly don't like it.

Earlier this month, when it seemed likely that Sprint would make a bid to buy outright control of Clearwire, one of Clearwire's largest shareholders filed a lawsuit in Delaware to stop any such deal. Texas investment company Crest Financial, which owns 8.3% of Clearwire, says selling off the company's large trove of spectrum would reap much more for its shareholders than a sale to Sprint.

"Rather than permitting Clearwire to operate as an independent company or conducting a fair process to sell either Clearwire or its assets for the benefit of all Clearwire stockholders," the lawsuit alleges, "Sprint ... launched a scheme to deliver unilateral control of Clearwire and its spectrum assets to SoftBank on the cheap, while extracting maximum benefit for itself."

SoftBank is the Japanese mobile operator in the process of buying 70% control of Sprint and whose cash infusion of $8 billion to Sprint has made possible the bid to buy Clearwire.

Crest has company in its opposition to the Sprint Clearwire deal. Mount Kellett Capital Management, holder of 7.3% of Clearwire shares, sensing Sprint's move, sent a letter to Clearwire's board of directors last November expressing his belief that "Clearwire's stock [is] substantially undervalued," and estimated that selling its excess spectrum could "generate gross proceeds of $6-$9 billion." The $2.97 a share Sprint is paying for full control of Clearwire works out to $2.2 billion. Current market capitalization for Clearwire is $4.2 billion.

For the deal to go through, Sprint needs support from shareholders adding up to 24.8%. It currently has 13%. So even though this merger would seem to be a no-brainer for Sprint shareholders, they shouldn't count their MHz just yet.