Verizon Wireless 4G LTE customers have been experiencing an outage since last night. The good news is that Verizon has discovered the cause and is working on fixing it.
Update 04/28/11 @ 9:20am According the the Verizon Twitter page the outage is over as 8:00am today. HTC ThunderBolt users should be back to normal service. Laptop users with USB modems "may need to re-connect to the network
when moving between 3G and 4G. This will continue to improve."
Yesterday Verizon said that all 4G LTE smartphones could make voice calls.
Verizon expects to see the network restored market by market. The Verizon Twitter feed updates the situation frequently.