ICS 4.0 (don't kill the messenger)
I'd look for the update 1st week of June. It's still in testing and they're about a week back from their "end of May" target, according to some very knowledgeable Lenovo folks here this week.
Another Android ThinkPad tablet is also not too likely.. they're still deciding, according to what I was told, but it's not really been a profitable unit in the scale they need to see to continue.
Gonna be a lot of Win8 devices this fall though.