Update (4/28/2011): Verizon has located the problem and is turning markets back on as we speak. If your service still isn't restored it should come back online sometime today
Verizon LTE 4G is currently experiencing a nationwide outage.
According to Twitter Verizon reports:
We're aware of an issue with #4G #LTE connections & our network engineers are working to resolve quickly. Will update here.
It appears that 3G is fine, so if you are experiencing an issue, be sure to report it at Verizon LTE Nationwide Outage