It's not so much speculation, but a logical conclusion considering how similar the Thunderbolt is to the Desire HD. The only real difference between the two are the radios (Thunderbolt: CDMA/LTE, Desire HD: GSM), and the Desire HD lacks a front facing camera. Everything else is virtually the same.
From the article linked to above:
"The decision is apparently in the hands of HTC at this point and we assume that means that the software is currently in testing. It was the testing of ICS on the HTC Desire HD that led to the device?s Ice Cream Sandwich getting cancelled and it?s possible that HTC will deem to software unfit for the ThunderBolt as well.
This is certainly possible considering both device share similar hardware ? including 768MB of RAM and a single-core processor ? something that originally led us to believe that the ThunderBolt update was in danger of not rolling out at all.
So, while nothing is set in stone, it appears that the ThunderBolt?s update is definitely in trouble and while there appears to be a little hope to cling on to, owners should start preparing to either root their device or endure a permanent life on Android 2.3 Gingerbread."
Personally, I'm going to call it now; no ICS for the Thunderbolt.