I was VERY surprised to see it appear on my HTC Thunderbolt today as well. I just got the phone a couple of weeks ago and was told then it wouldn't be until July.
It remained in 4G mode all the way from Stamford to Citi Field tonight. Speeds was as previous poster described. Very sweet.
I have had the Thunderbolt for a few weeks now. I live in Stamford and noticed this morning that I now have 4G service. Just happened! Way to step up Verizon.