Credit Suisse analyst Jonathan Chaplin estimates Clearwire will need another $4 billion to extend coverage to the 200 million people they plan to reach by the end of 2011.
That is the biggest carrot for Clearwire: it needs cash, and T-Mobile could provide some of it.
Though Sprint might have qualms about enabling a competitor, Verizon and AT&T, not T-Mobile, is the big problem.
When Sprint and Clearwire merged their networks, Sprint invested $1.2 billion in the venture. Comcast and Time Warner invested a total of $1.6 billion, and they now market Clearwire's service under their own names.
Intel put in an additional $1 billion on top of the $660 million they had invested in Clearwire earlier.
Google invested $500 million and cable operator Bright House Networks kicked in $100 million.
Whether Clearwire gets T-Mobile USA as an investor or not, nothing is going to keep T-Mobile USA from finding some way to provide 4G services. The lesser of the two evils might be to allow T-Mobile USA to invest in, and use, the Clearwire network.