Update:

> there seems to be progress on Nanojit/TraceMonkey for PPC

It got released, the first PPC web browser with a JS JIT:

"After working more with the nanojit, I have decided not to make it the default, but it is there, and you can use it."
http://tenfourfox.blogspot.com/2011/01/beta-9-now-available.html

"The first draft of the PowerPC-specific nanojit is now available, helping to accelerate certain JavaScript operations. This portion of code is specific to TenFourFox and is still under development, so it is not enabled by default."
http://code.google.com/p/tenfourfox/wiki/ReleaseNotesBeta9

"As of beta 9, 10.4Fx uses a custom PowerPC-enabled nanojit for JavaScript acceleration through TraceMonkey. Currently 10.4Fx is the only Mozilla-based browser to use this module, as it was developed for it; it is being contributed back to the Mozilla codebase for other Mozilla source consumers."
http://code.google.com/p/tenfourfox/wiki/TechnicalDifferences

"Thanks to user PoLiYa who wrote in to tell me about his fabulous score on SunSpider with his PowerBook G4/1.5GHz and the nanojit turned on: a whopping 2866ms, down from around 6400ms. [...] I got out my iBook/1.33GHz and repeated the test. [...] On the G4, the nanojit not only worked, it easily cut SunSpider scores in half, and Dromaeo was also about double the speed. User agg23 over at 68KMLA tried it on his G3, and also saw a similar speed improvement, from 16000ms down to around 8900. [...] the plan for beta 10 is to ship the G3 and G4 versions (7400 testing is pending) with the nanojit turned on for content (web pages). If this works, it may also be enabled for chrome (i.e., the browser's own JavaScript, and add-on JavaScript)"
http://tenfourfox.blogspot.com/2011/01/lies-damn-lies-statistics-and-g5-beta-9.html

Nice comparison chart:
http://www.floodgap.com/software/tenfourfox/graf.jpg

[ Edited by Andreas_Wolf on 2011/1/18 11:36 ]
[ Edited by Andreas_Wolf on 2011/1/22 0:49 ]
[ Edited by Andreas_Wolf on 2011/2/8 3:21 ]