$ mzscheme --version
Welcome to MzScheme v4.2 [3m], Copyright (c) 2004-2009 PLT Scheme Inc.
Also, I second the notion that shared-memory threads are a bad idea. I really like Termite's message-passing model.
Which test were you running, akkartik's or palsecam's?
This is what happens with palsecam's code:
arc> (= var 0 tbl (obj var 0))
#hash((var . 0))
arc> (repeat 50000 (thread (++ var) (++ tbl!var)))