Merge is basically the Desire Z
ThunderBolt is the Desire HD with upgrades and different design slightly
It may still receive the Droid branding for both the Merge and ThunderBolt
the ThunderBolt was last seen in the Verizon inventory as "Droid ThunderBolt 4G" with a retail MSRP of $615.00.
and if you recall the branding change at the last minute last year with the Incredible, it was originally going to be HTC Incredible and the website first showed that, then a few hours after it was changed to say "Droid Incredible by HTC"
the droid branding wasn't mentioned at the launch event when it was announced.
the screenshot posted on AndroidCentral was updated on 2/11/11
and here's a comparison of photos (first one is the ThunderBolt, second is the Inspire 4G on AT&T, its identical to the Desire HD which also allows it to use the same cases/accessories as the Desire HD in europe)