I have only been seeing a 1X connection on my HTC ThunderBolt all morning and after coming to work and finding a couple others with the device experiencing the same I posted to my Twitter account. I also have a new Droid Incredible 2 to evaluate that shows a solid 3G signal. My friendly followers confirmed there is a LTE outage on Verizon at the moment and the way that the ThunderBolt handles data is different than any other device on the network so it is not picking up 3G. While you wait for LTE to come back, you can perform the steps described on Android Central to at least go back to a standard 3G connection.
The HTC ThunderBolt uses eHPRD to provide both voice and data over 3G and 4G on Verizon and this is the default setting. The steps linked above switch you back to standard Rev A 3G mode so you can get faster data than 1xRTT that you will see by default when 4G is down. Verizon's Twitter account states that engineers are working on it so it sounds like this wasn't a planned network upgrade or anything. I was hoping it was since I have had horrible performance with their WiFi hotspot functionality performing longer than 20 minutes or so.
I also read that Verizon may extend the WiFi hotspot promotion past the 15 May date that was stated when I purchased my ThunderBolt. There is currently no pricing plan for LTE WiFi hotspot capability and it seems quite a few of us are having LTE hotspot issues so I think Verizon has more work to do before they can start charging for a broken service.