Originally Toshiba was planning to release the Ice Cream Sandwich update for the Thrive and the Thrive 7 sometime in the Sprint. Unfortunately, not only did they miss that target, but they won't even make the Summer either. Apparently, Android 4.0 for these two tablets has been delayed until the Fall. Toshiba did issue a statement about the issue, but the statement did not indicate the reason for the delay, nor anything more specific than Autumn. Here's the statement,

We are working hard to bring Android 4.0, Ice Cream Sandwich to our Thrive tablets. While we originally estimated it would be ready this spring, we now estimate the update will be delivered to all Thrive 10" and 7" tablets by early fall. Apologies for any inconvenience and thank you for your patience.
Source: ThriveForums.org via Phandroid