What is it? From Gizmodo last week:
" - T-mo will be pushing that 21Mbps HSPA+ data service they just test deployed in Philadelphia, nationwide. Speculation has the nationwide roll out of T-mo's 21Mbps HSPA+ to be largely complete by the halfway point in 2010. So pretty much if you live in a larger metropolitan region already covered under T-mo's network, by next July you should be able to take advantage of the new speeds.

2 - As part of Project Dark (or Project Black, depending on the source), roll out of their 21Mbps HSPA+ network will accompany a new all-you-can-eat price plan that includes voice, data, SMS and MMS. Speculation once again has the price plan firmly set at $50/month.

3 - Finally, Project Dark will be launching with new 3.5G smartphones outfitted to take full advantage of 21Mbps HSPA+. The initial line-up of 21Mbps HSPA+ smartphones include two Android phones, a Blackberry and Nokia's new Maemo 5 powered tablet:

Blackberry 9700 (Bold 2)
Motorola CLIQ
Samsung Behold II
Nokia N900 "

However, on Friday 10/16, this rumor emerged claiming, "three independent, possibly insider sources" claimed a 4G-capable iPhone would be added to the handset selection and we'd see the initial launch of said, "Project" on 10/25:

http://nexus404.com/Blog/2009/10/16/t-mo...

No matter what, I suspect, "4G-capable iPhone" is COMPLETELY unrealistic, but, hopefully a $50, "Everything" plan is for real!