Mobile World Congress helped to flesh out more details on US operators' plans for initial 5G towns and cities and suppliers.

So here's where we're at now:

AT&T
What: Mobile 5G using the 3rd Generation Partnership Project (3GPP) 5G New Radio (NR) standard, with fixed 5G in trials
Where: Atlanta, Dallas, Waco and nine other unnamed markets
When: Late 2018
Frequencies: AT&T hasn't confirmed but 39GHz is widely rumored
Suppliers (known so far): Qualcomm
Devices: A mobile "puck"
Capex plan for 2018: $23 billion

For more:
AT&T Reveals Initial 5G Cities
AT&T's 'Mobile' 5G: What the Puck?
AT&T's Mobile 5G Plan Leaves as Many Questions as Answers

Sprint
What: Mobile 5G using the 3rd Generation Partnership Project (3GPP) 5G New Radio (NR) standard
Where: Atlanta, Chicago, Dallas, Houston, Los Angeles and Washington, DC
When: First half of 2019
Frequencies: 2.5GHz
Suppliers (known so far): Ericsson, Nokia, Samsung, Qualcomm
Devices: Smartphones
Capex plan for 2018: $5 billion-$6 billion

For more:
Sprint's Initial 5G Cities Will Get 4G Massive MIMO First
Sprint Teams Up with Cox to Boost Spectrum With Fiber
Sprint Says No to mmWave, Yes to Mobile 5G

T-Mobile US
What: Mobile 5G using the 3rd Generation Partnership Project (3GPP) 5G New Radio (NR) standard
Where: Dallas, Las Vegas, Los Angeles, New York City and 26 other cities
When: 2019-2020
Frequencies: 600MHz, 28GHz, 39GHz
Suppliers (known so far): Ericsson, Nokia, Qualcomm
Devices: Smartphones
Capex plan for 2018: $4.9 billion-$5.3 billion

For more:
T-Mobile to Roll Out 5G in 30 US Cities in 2018
T-Mobile Promises 'Nationwide' 5G in 2020 With New Spectrum
T-Mobile Says Its 600MHz LTE Sites Will Be 5G-Ready

Verizon
What: Fixed 5G using its home-grown 5GTF spec first, with 5G using the 3rd Generation Partnership Project (3GPP) 5G New Radio (NR) standard as soon as possible afterwards
Where: Sacramento, with up to four more markets to follow in 2018
When: Second half of 2018
Frequencies: 28GHz
Suppliers (known so far): Ericsson, Samsung, Qualcomm
Devices: 5G customer premises equipment and home routers, smartphones (with mobile 5G)
Capex plan for 2018: $17 billion-$17.8 billion

For more:
Verizon: We'll Be First With Fixed & Mobile 5G
Ericsson & Samsung to Supply Verizon With Fixed 5G Gear
Verizon Capex to Stay Flat Despite Commercial 5G Launch in 2018

What does this mean in the real world?
US operators are busily getting ready for the expected arrival of mobile 5G with smartphones coming as soon as the first quarter of 2019. (See A 5G Smartphone Timeline for 2019.)

— Dan Jones, Mobile Editor, Light Reading