TraceMonkey was announced on 2008-08-23, after 2 months of development[1].
V8 had its first public release together with Chrome 2008-09-02 as you already mentioned, but development appears to have started in 2006 according to some of the copyright notes in the initial SVN export[2].
Yes. I did not take Bryan to task on that, but it really gives V8 a bit too much credit for it to cause Andreas Gal to work on trace-JIT before 2006 (on Java, for his UCI PhD; then on JS in collaboration with Adobe and Mozilla).
Tracing was a good rocket to strap on SpiderMonkey-the-2008-era-interpreter but it fell to a combination of the PIC-based approach V8 championed and Brian Hackett's Type Inference work (PLDI 2012, http://rfrn.org/~shu/drafts/ti.pdf).
V8 had its first public release together with Chrome 2008-09-02 as you already mentioned, but development appears to have started in 2006 according to some of the copyright notes in the initial SVN export[2].
[1] https://brendaneich.com/2008/08/tracemonkey-javascript-light...
[2] http://code.google.com/p/v8/source/detail?r=2